First thing for you is to realize there are LOTS more families with in-laws who don't like each other than those who do. There are thousands of in-laws doing the "tippy-toe" dance around each other. Sometimes, the problem is when at a family get together, we are out of our home zone where we are in control. Sometimes, it is a situation where the problem in-law(s) are just not likable. It seems too many of us make it a competition of who is best and has the best. The most petty things become huge. Drives me nuts. It stands to reason that each of us has our own household, lifestyle and way of doing things.
My husband and I married 5 yrs ago(2nd for me)late 40's couple. My husband has a BIL who is a loud mouthed, know it all, lucked up in money, verbally abusive, Christian rejecting, jerk. SIL over the top evangelical, holier than thou, nothing but designer. Dad also "christian" who raised the kids without tv because it was evil, who now spends 12 hours a day watching the tube, verbally abusive. All the clan including aunts,uncles, cousins and the once removed descended on us(univited) for 3 days a few months after we married. They somehow got together and decided on this 3 day family reunion at my house and then informed me a week before it was to come off. Everyone wanted to see the divorced woman with 3 grown kids who finally caught the bachelor of the family and put her to the test. It was a nightmare. On day one, I quickly got enough of the snide comments, bragging, down the nose looks at all around. As we were all in close proximity, after one too many comments from the "christians" I replied that all have sinned and come short of the glory of the lord and let he without sin cast the first stone, if thy brother has offended thee, go to thy brother in private. There were about 25 people there, the place went silent, you could have heard a pin drop for about 30 seconds. I had addressed the room at large, but everyone knew who the targets were. That cured it. Evidently, no one had ever crossed any of them before. BIl suddenly remembered he had to get started on the 6hr drive home for some forgotten chore and as he had made the wife drive separately anyway, she could leave when she wished. SIL was a little more discreet, waited about an hour before announcing their family should get going on their 4hr journey another direction. Neither of them returned the other 2 days. Little harder for dad, as they were staying in a motel 2 miles away and most of the company in the house were his relatives. Took him a couple of hours to announce he was just tired and needed to rest for a while. He had no choice but return the next 2 days and was civil and every once in a while, charming.
Only been 2 other gatherings since then. All 3 greet and then try to avoid me. If I see they have someone cornered, I just walk up and ask if I can join the conversation. I didn't change them, just changed the way they speak in my presence. None of the 3 like me. Everyone in the family knows it. I'm fine with that. I don't like the way they treat people. Really, what are they going to do with someone like me? I don't criticize, accuse or attack, just let them know in terms I know they understand their behavior is offensive to all in attendance. Oh, they could shut me up, cause me to leave, but not without making themselves look like REAL jackasses in front of the others. All 3 have issues that seem to require hurting other's feelings. Not my problem, not going to let it become my problem or ruin an otherwise enjoyable gathering. I adore my husband's sister married to the jerk. My husband's mom is such a gem. Don't care for the brother married to the ultimate christian, but hardly have occasion to speak with him,anyway. I have an excellent relationship with both of my husband's sisters and mom. We speak on the phone, by email and letters.
I am fully aware that all 3 "on their list" in-laws have said things to the effect that I think I am better than them. In one respect, that happens to be true and I will make no apology for it. I am better, in that I see no need to belittle or humiliate others. I see no need to try and prove I am better than anyone else. I have as much "baggage" as anyone, but when I am at a celebratory gathering I leave that at home. If it takes making someone else feel bad in order for me to have a good time, then I should stay at home. So, yes, in that respect, I think I am better and should the day ever come any of the 3 says this to my face, the above will be my answer.
In your situation, we will assume that both BILS are just not likable, as you state that you don't care for them any more than your husband. If that is the case, both your sisters are probably in the same situation as you. If their husbands are the not likable, don't like any of the family, got to flex the muscles and beat the chest at gatherings, they probably don't like your husband, or each other. Even though, they may join forces at gatherings and appear to get along.
You might try approaching the subject by opening a conversation in the more general terms of how uncomfortable things can be at a family gathering. Then, address that your husband feels her husband doesn't care for him very much. Be prepared, and open to the possibility that her husband feels that you/your husband think you are better than he. DO NOT tell her any of your specifics, even if she tells you theirs, lest you and sis get into a hard to repair argument. Remember, you brought this up, her husband may have his own list. Go prepared to hear things you didn't know and would probably rather not know. Try and just listen, then you'll know what's up. When she's finished the two of you can agree it's better to keep them apart. Again, don't recite your list of what's wrong with her husband. Let what she said about yours be forgotten. Even if she asks you to clue her in on what you think is wrong with her husband, don't go there. Your goal was to find a way to maintain the relationship with your sister and not have to be in company of her husband. Listening to a litany of complaints about yours while keeping silent about hers may be the price you have to pay.
Or, you could try to keep it in the generalities of how the subconscious carries issues of sibling rivalries and childhood issues into adulthood and surface when in larger groups, even though those present are not siblings. How men, whether they think they are or not, are doing the "survival of the fittest", "leader of the pack" thing. How all of you sisters chose men with strong, leadership personalities and those things come into play and what you, as sisters must do to keep these things from coming between you. Even if it means not getting together as foursomes.
If it is that your husband is not the outwardly competitive type(like myself and my husband) he still competes to have the outwardly competitive leave the competive nature at home when he's around. If he is the outwardly competitive, he may not like being around a guy who appears to him to be too passive. Maybe the guy is alcoholic or spendthrift or something more concrete to dislike.
While sis's husband may be a jerk, it could also be that he really does feel inferior to your husband. Whatever the problem, you have to be aware that as your feelings toward BIL are reflective of your husband's feelings, so are your sister's reflective of her husband toward yours. If you are going to be offended to find that out, I would leave the situation as is.
Seems to me that men some men kind of love hating each other. Whether it's the golf links, the bowling league, the softball team, the pickup football game. They go back week after week no matter how mad they get at other guys. These men evidently have no understanding of men who just don't want to be like them and don't want to be around that kind of atmosphere. While their competitive buddies tick them off, these non-competitors scare them, which ticks them off. I don't know what is driving the dislike for your husband, but the things I have mentioned are just a few of the possibilities. It doesn't really matter what's driving it, the issue is he doesn't want to be around them, but if you know the cause, could be very helpful in how you discuss it with your sisters.
This is your sister. I would be pretty sure she is aware of the undercurrent of dislike among the men. So, my advice is to start a conversation on the more general terms of family discomfort at holiday gatherings, see her reaction to that. Then maybe progress to an experience of seeing aunt or uncle so and so who couldn't seem to get along, or maybe how mom or dad felt about one of their in-laws. Tell sis you are so glad that your relationship is a strong one. Talk about how guys are guys whether they are surfer dude, construction worker, or corporate executive. I mean, don't we all know a guy with a huge paunch, bad combover who wears a faded hawaiian shirt, speedos, white knee socks and sandals and thinks his date should be 36-24-36, perfectly coiffed, maniucured, designer dress and spiked heels???????
And what do you do if you discover her hubby really likes yours and would like to spend time as a foursome? Time for you to pony up and realize the issue is his/yours, for whatever reason. If that's the case and you are not willing to admit it and go there, best leave things as they are for another while until you are comfortable with letting sis know how you feel and the consequences that may follow.
There's two cents of my nickel's worth. Hope it at least gives you food for thought. Good luck.