Who throws a bachelorette party?

I'm in a wedding party and am probably the lowest bridesmaid on the totem pole, but also one of the oldest. Two brides, 12 members of the bridal party - they're putting boys on one side and girls on the other, but each bride has a people on each side. My friend-bride's maid of honor is her little sister. I have never been to a bachelorette party and feel like this is just the couple I would love to throw a celebration for. But, I feel like it is not my place to try to put something together. And even if I would, would we do a joint bachelorette for everybody? The little sister isn't over 21 yet so I don't think this is something she can do.

Is there an appropriate etiquette here? Should I just butt out?