I do not get along with my fiance's sisters. Do they have to be in the wedding party?
No, they don't have to be. This issue -- having your intended's siblings of the opposite sex stand up for you -- really depends on the specific family. In some, it's completely expected and they'll be mortally wounded if you don't ask. In others, unless you're best friends, they would be surprised if you asked (it might be nicer for them to spend the day with their family than with your attendants). Of course, just because his family (or yours) expects the wedding to be any certain way doesn't mean it has to be that way.
If you don't currently get along well, not asking his sisters could make your relationship worse if they feel snubbed. Like it or not, they're going to be your sisters-in-law very soon. If you really don't want them as attendants because you want your sisters and close friends instead (and rightfully so, especially if you're having a small wedding party -- you can just explain it that way), have them do readings during the ceremony or stand up on your fiance's side. These days, your sex no longer defines whether you're a bride's or groom's attendant. They can wear the same dresses as the maids, dresses they choose themselves, tuxedo-inspired outfits, or even tuxedos, depending on everyone's sense of humor.
The bottom line: There's no rule that his siblings have to be in the wedding party. But leaving them out might not be a good precedent to set.
Most Asked Wedding Questions
Choosing Wedding Attendants
Your Bridal Party