Bridesmaid + her younger sister

So my FH has 3 sister. His older 2 sisters are from his dads side and his 3rd oldest sister, we'll call her sister "A", is from his mom's side. Sister "A" is close with me and she is a bridesmaid in the wedding. (Sister "A" is also FH's mother's perceived favorite, been in this family nearly 10 years, i already know it.) Sister "A" is always bringing HER little sister, from her father's side of the family, to events and what not, which I'm okay with. I do like "A's" little sister and don't mind her being around. However...
I'm feeling mixed signals from all around, FH, FH's mother, about her being invited to the wedding. Now I stand firm with not inviting her because the main reason everyone wants her there is to watch my 1 yr old nephew who is also in the wedding and is "A's" son. Now sister "A" has a husband and because sister "A" is my FH's sister, nearly half her mothers side of the family plus my own family will be there. I don't think its acceptable for sister "A" to basically have a personal baby sitter for my nephew whereas anyone else who has children doesn't get that "luxury." Now I understand sister "A" is a bridesmaid and she won't be able to tend to her son, but my FH's mom will be there and other people as well, within our "inner circle." We've already cut nearly half of our family that could have been invited but didn't to keep the wedding on the smaller side and we also nixed plus ones. My FH's mother didn't actually help us make a decision about "A's" little sister she basically said "its me or A watching the "baby" thats all she said. It feels like she doesn't get our view but is just making it easier on her and her daughter. Now we did toy with hiring a sitter for the wedding but because FH's 3 neices live out of state, HI, its 50/50 shot if they'll come and the only other little, under like 8, is my little cousin.

What should we do? We're trying to keep the guest list between 95-100 or even less than that and be courteous to those family members not invited. (Also sorry for the long post 🙂)