Wow - I'm in the same camp with Jen M. I'm that sister in law. I know you've gotten a ton of advice here (some of which is appaling to me, and they should be ashamed of themselves!!), so I'll try to keep it short here.
1. You are only seeing things from your side. I would bet a million bucks there's been some hurt feelings for your sis-in-law.
2. You try to make your brother choose or confront him with out sis-in-law there, you will be sorry. Either your brother will choose his wife, as he should, or you will divide their marriage, which is unaccepatble.
3. You need to get new friends - ones that don't spread gossip. This is a big reason I have trouble with my inlaws - they listen to and spread gossip.
4. You need to relax. You said your self that she has a larger family than you, and yours is very large. They are probably getting pulled by everyone. There is no way to divide time so that everyone is happy, and the more you pull them, the more she will resent you. And your brother eventually will too, if she's upset, he'll hear it!
One of my big peeves with my in-laws is this: they don't understand extended vs. immeadiate family. For the first 2 years of our marriage, my husband thought his aunts & uncles and grandparents were all immeadiate family. This is nonsense! It's crazy! We had to redefine family. There's family of origin: parents, bros & sisters; extended family: aunts, uncles, cousins, grandparents; and there's immeadiate family: husband, wife, & kids. When you get married, your spouse & kids come first, family of origin second, and extended family third.
And with a family as big as yours, you have got to realize that they can't be there all the time. With eight married siblings, that's 16 birthdays a year, not counting your parents, your kids, cousins, etc., plus her even larger family. It's too much. Cut them some slack. Who knows, your brother may be getting the same bad rap from her family... that they always pick you over them! There are just too many demands on one couple.
Either confront them together, one couple to one couple (NOT eight couples to one couple), Or... get over it.
Good luck.