Sweet Sister (1978)

Miss Cassidy is the titular Sweet Sister whose hapless and horny brother, Jamie, writes to her from college complaining that he can't get laid. Sis, of course, is an absolute expert in getting laid, banging both her boyfriend, Jim, and her best girlfriend, Katy, before her virgin bro arrives on the scene.So, to help deflower her brother in style, Cassidy throws "a party at the cabin" where Katy, Jim and another gal make sure Jamie doesn't go back to school with his cherry. In the end, Jamie is transformed into a stud, returns to college, and fucks the shit out of his chubby little coed girlfriend.