How can I quit a band without offending them?

ok, i started in this band about 2-3 weeks ago. I really don't want to be in anymore, i dunno how to tell them i wanna quit because theyre family friends-I can't say a relative is sick. and also theyre in the same classes as me so i cant say it is homework, theyre getting me lessons nd everything n want me to do gigs n i dont want to. i cant say i dont want to because if i quit, they cant carry on the band because nobody will fill my space. PLEASE!!!!!!