This is a charade - multiple word parts are joined together in sequence to form the answer.
Break down the clue: 'Familiar with' gives you a 4-letter word meaning knowledgeable about something, and 'your sib's butt' gives you a 3-letter word for the rear end of your sibling. Put them together in order.
Answer: PERTURB | 'Familiar with' = PER (knowledgeable about), 'your sib's butt' = YOUR sibling's rear = UR + B = URB, giving PER + TURB = PERTURB (trouble)