Can I ask one sister to be a bridesmaid but not the others?

I don't believe the bride and groom should have each other's siblings stand up on opposite sides of the wedding party. If you want your brother in the wedding, have him stand up on your side. You can call him 'bride's attendant.' If your fiance wants his sisters in the wedding, he can have them stand up on his side as groom's attendants. That puts him in charge of dealing with them and figuring out what they should wear, if you don't want to deal with them on subjects like that. Keep in mind they may or may not host a shower or bachelorette for you though, and you'll have to prod your brother about the bachelor party a little probably.