Bridesmaid Issue

I just got engaged a month ago and booked my venue for July 2017. Other than that, that's the only decision on paper. My fiancée and I both know who we are going to ask to be in our wedding party, but haven't formally asked yet. Eight on each side. On my side, I have a lot of close friends, a sister, and a cousin I'm really close to. My cousin has a sister who is 17, and is assuming she is going to be asked. I honestly didn't consider it, as we are not that close, she's going into college, and I don't think it'll be financially possible for her. Well, my bridesmaid cousin, told her mother that I didn't consider it, and now my Aunt is saying that my little cousin should stand up in the wedding. I'm very confused as to what I should do about this because I don't want to hurt my little cousin, but I don't want to ask her to be a bridesmaid. Advice?