Is it necessary to ask my fiance's brother or sister to be an usher or bridesmaid?  I have a close friend who I want as bridesmaid and I don't really feel comfortable asking.

While it's a nice gesture, it isn't a command performance.  It's more important to choose friends or relatives you wish to honor and who you know will take the responsibility seriously.  Being an attendant requires a major commitment of time and expense, they're expected to help with wedding planning, provide emotional support and purchase their own attire.  If you do choose to use these attendants, what better way to honor them with a personalized gifts.