I'm sure this is somewhere on TK, but to be frank, I don't feel like stepping into the general boards, where responses are sometimes less-than-friendly.
FI has two sisters. One is my age, divorced, with a son. That son will be in FI's party (junior groomsman or something like that). The other is in her mid-40s, married, with two children. Those children will be a flower girl and a ring bearer.
Am I obligated to ask the sisters to be bridesmaids? I'm friendly with them and like them a lot, but don't consider them my girlfriends. I'm definitely more comfortable with the one who is my age.