TBH, felice, I understand a bride wanting a babysitter for her kids, so she can just enjoy her wedding day. I'm sure baby got plenty of attention in those 2 weeks. And I wonder, was it about "attention" or babycare? I don't know your XSIL, of course, but generally speaking, I would understand a bride's wanting a childcare break for just that one day.
What I don't fully get is her turning to your mother/her then-BIL's wife's mom for help! But I suppose her own mother and BIL's were involved with the wedding.
Also, I don't get why XSIL didn't have DS1 stay for the meal. But perhaps he was getting restless? (Hard to know since you weren't there, I realize.) It sounds as if there was some miscommunication between XSIL and your mother about when he would arrive. Anyhow, I feel sorry for the poor little guy being sent back and forth - nobody wanted to be bothered with him!
It's not my business, I know, but I don't get why you didn't accompany your then-husband to the wedding. Just cuz XSIL ASKED your mother to watch her baby? Maybe I'm wrong, but it sounds to me as if you just didn't/don't like this lady, in general, not just cuz of whether or not she had her kids at her wedding! (I realize you may have good reason to dislike her, of course.)