College kids... ehhh.. pretty irresponsible self-centered folk, a lot of the time. They may not have ever been party to such a momentous occasion and just didn't "get" what a big thing this was for her. And, they might have been tired or hung over and assumed that everyone else was going, so it was okay for them to miss.
Your friends, same thing. Not the irresponsible part, but they probably assumed that all her college gal pals would show and they'd be the ones left feeling a bit awkward. So missing wouldn't be a big deal. I doubt they PLANNED to no-show after saying they would come... but maybe they really did forget what day it was. I have had times in my life when I was so distracted with other things that I would have forgotten something like that COMPLETELY if my hubby hadn't remembered, or one of the kids mention something off hand like "when is that party you were talking about?". OR, maybe they were busy and kept putting off the gift shopping long enough that didn't have it bought by the day of the shower (this is probably more likely... it's happened to me) and had to stop on the way if they were going to come. I have been in some pretty tight time frames trying to buy gift bags and tissue to fit a just bought gift in the car! But maybe they got behind schedule and didn't have time to stop and pick up a gift, so they skipped it all together.. again, thinking that all her college friends would be there so no biggie.
But, I am curious about your comment "this is not the first time something like this has happened.". How so? That you threw a party and no one came, or that your best friend ditched you after saying she would show for something? You probably are more upset because no one showed, than you would be if all her college friends showed up and just your friend didn't. But, is this a pattern of your friendship? If so, then she didn't do anything out of character so you shouldn't be fuming. You knew she had this tendency, and you have never felt the need to call her on it, until now. I would be very careful, if you bring it up to her, that you don't let it spill into areas you don't intend. For her, sounds like it was just the status quo. She won't get why you are SO upset. I think you should take a step back and examine your entire relationship with this woman, and decide if that is the kind of friend you want to have. Are her other qualities sufficiently important to you that you really don't mind her ditching you sometimes? Then don't mention THIS time. If you are fine with that. Then be fine with it. If you are not, then be prepared to lose the friendship, because this will disrupt the entire balance of your relationship. It doesn't mean that you WILL lose her as a friend, but anytime you alter the basic balance it is a possibility. She may not adjust to the "new dynamic" and drop you. So just be prepared for that possibility.
As for you SIL, I'm sorry she didn't have more attendance, but that you guys had to the good sense to enjoy what you had and not sit around being bitter all day and making it all a waste. Like it or not, she just got an eye-opener about parenthood. MOST people in this world aren't going to be looking out for your child's best interests... that is YOUR job. And sometimes it is a lonely place to be. Totally worth it, of course, but she knows who cares about her child now, and it obviously isn't her college friends. Kudos to you for working so hard to give her a nice shower and welcoming your new niece!