What is the name of Abby Cadabby's stepbrother?

Abby Cadabby’s stepbrother on Sesame Street is Rudy.

Rudy is a young orange monster with blue hair who joins Abby’s family as part of a blended-family storyline. Abby’s mother, Maggie, marries Rudy’s father, Freddie, making Abby and Rudy stepsiblings rather than biological siblings. Their relationship gives the series a way to discuss changing families, sharing space, and working through disagreements.

Rudy was introduced during Sesame Street’s modern era and is portrayed as a curious, energetic three-year-old. He enjoys animals, especially dogs, and often gets into trouble when experimenting with Abby’s magic wand. The character is performed by puppeteer Frankie Cordero.

Viewers sometimes confuse Rudy with Freddie, who is his father and Abby’s stepfather, or with Telly Monster, another recurring Sesame Street character. Murray is also a former Sesame Street Muppet, but he is not related to Abby. The canonical answer to the question is therefore Rudy.