What generous family members you both are. I suppose I would sit down with your husband and have a no-nonsense conversation with him about specific reasons why he does not want to pay for your SIL's airline ticket, if he is willing to pay for every member of his family. To me, saying he is willing to pay for your Brother, but not your SIL shows that he doesn't consider her family. So in his mind, are in-laws not family? Would it be acceptable for his parents, or his brother to get him something for Christmas, but not you, because you are just an in-law and, therefore, not family according to that reasoning? Or, are they capable, like your parents, of paying for their own seats, so he might feel taken advantage of by people who can pay, but don't want to? OR, does he have a specific reason to not like your SIL?
OR, is there something else going on......are they willing and able to pay for their own tickets, but you feel like you do more for his family than yours and you are trying to make a point with this issue. (if that is the case, trust me, I am not judging you, but I would suggest you work through the actual problem rather than going tit-for-tat over sizes of gifts, which might be a symptom, not the actual problem)
If it really is about wanting to help your brother's family and if you really feel like you should pay for her as well, then perhaps make a list of everyone who has come out to see you, both your family and his, along with dates, and who has paid for the tickets and for what reason. Since you probably won't be there long enough for an additional visit from your brother and his family, and you have said you have not purchased any tickets for your family yet, perhaps make a deal with your husband. Tell him that since you have purchased 4 tickets as presents for his family, you feel that it is fair to have the option to purchase up to 4 tickets for your family as Christmas presents......be they in-laws or not, they will just receive their Christmas present early. That may take keeping things even to a drastic point, BUT sometimes it is the only way to make sense to people and keep hurt feelings from coming in to play. Don't feel bad about them receiving their presents early, my family does that kind of stuff all of the time, and it always works out great because everyone knows the rules...just make sure that you stick to it as their present and don't get them anything extra so they have something to "unwrap."
Sorry for the long response, but the issue of keeping things even between families is something I have run into, time and again for 12 years, and it is exhausting. Hopefully you can get it figured out much sooner than that in your own life! :-) Good luck!