The contents of this article are based on an issue of Simpsons Comics or another comic series and is considered to be non-canon and therefore may have not actually happened/existed.
Patty & Selma's Ill-Fated Romance: My Sister, My Homewrecker! is a comic story featured in Simpsons Comics 2.
Can it be true? Marge's sister Selma is having an affair with -- Homer?
Marge is unable to believe her ears when she hears Homer drooling and muttering Selma's name in his sleep. She is even more disbelieving when she observes Selma kissing and caressing Homer. Looking to Patty for comfort, Marge gets up the bravery to confront Homer during another date with Selma and punch his head off his shoulders... literally.
Dr. Marvin Monroe enters to explain what is happening. In order to help Selma find a truly-happy relationship, Monroe suggested that she spend time with Homer, whom he feels is the root of her problems in finding love. Naturally, Selma refused, so Monroe had an animatronic copy of Homer made for Selma to date and break up with, which she was just about to finish doing before Marge ruined the entire treatment.
Even though Marge is disappointed that she caused problems for her sister, she is at least pleased to understand that Homer is not cheating on her. However, he is still moaning about Selma in his sleep... specifically Selma, Alabama, where a big all-you-can-eat ribs festival is going to take place.