Who do I pick for my maid of honor?

I only have three bridesmaids picked out for my wedding, but they were the closest women I could find: my cousin, my best friend from high school, and my fiance's sister. I don't really know any other girls, so I thought I would be safe by choosing them.... Initially I thought I would make my cousin be the maid of honor, but I'm having serious second thoughts. I really don't know what she would have to say about me, or my husband. She is 27 and I am almost 21 and she has known me basically all her life. However I can't imagine what kind of a story she would have about me as I am not that close to her. I unofficially (haven't told her yet) picked her, because I thought the maid of honor should probably be the most mature and responsible, and good with people. My best friend from high school would have some great stories to tell about me but I don't know how she would feel about standing up and giving a speech in front of everyone. She is 21, just like me. Who do I choose? Can I opt to not have a maid of honor? Do I let the girls decide? What if nobody wants to do it? My fiance will have a best man for sure, so I don't know what to do if I don't have a maid of honor. I am not really close to anyone but I have already asked the 3 girls to be bridesmaids and they said yes, so I can't exactly back out now..