Its your wedding and you should do what you want. I have twin brothers, who are 1 year older then me. They are my bestfriends. I had them as my "bridesmen" with my two best female friends. My MIL as they all do had, a few opinions like why can't they be groomsmen, why do they have to be "bridesmen" but my now husband and I said that this is how we wanted it and that is how its going to be. We don't care what "tradition" says, its our wedding and did it how we wanted to. My brothers were so honored be by my side and everyone thought it was great. Do what you want to do, its your day...bottom line.
I think it's totally ok to have bridesmen, and groomsmaids. I was a grromsmaid a few years ago (totally awesome!) and will be having a bridesman in my party for my wedding next year! It's all about who is closest to you and who you want at your side on the big day. All the best!!