Wendy's brother (unknown name) is first mentioned in the episode iMake Sam Girlier. Wendy says that Sam took her bra, stuffed it with pudding and put it in the freezer. Sam then threatened to beat up Wendy's brother, forcing him to give his muffin to her.
Wendy mentioned her "little brother" again in iTake on Dingo. She saw the Dingo Channel ripping off iCarly bits when he was watching Totally Teri.