Wedding Party Conflict

My fiancé and I are having a difficult time figuring out who we want in the wedding party. I have 4 solid choices for bridesmaids, but am not sure if I want to include my 2 cousins (who are sisters). The day of the wedding one will be 17 and the other will be 12. Is that too young? It feels like I can't just choose the older one. Are junior bridesmaids still a thing?

As for the groomsmen, he has 3 solid choices in mind, but after that he has 9 or 10 friends he's equally close with. He also has his only male cousin, who will be 17 at the time of the wedding, but has severe social and behavioral issues. This cousin's sister is also one of my bridesmaids (she's my age).

We recognize that having more people may be a bigger expense (rehearsal dinner, gifts, etc.), but we also want our close ones close to us, but don't want the tables tilted too heavy on either side.

ALL help is appreciated!!