In all probability, yes, even though it's never directly stated.
In the episode "The Regina Monologues," the Simpsons visit England and Grampa looks up Edwina, a woman he had a brief fling with while he was stationed in England during World War II. It turns out that Edwina has a daughter named Abbie, who was born just under a year after Grampa left.
Even though it's never stated straight out that Abbie is Grampa's biological daughter (and therefore Homer's half-sister), it's pretty obvious that she is. First, the timing is perfect (her date of birth vs. when Grampa had the fling with her mother); and second, Abbie looks and acts just like a female version of Homer. Think of Homer about 15 years older and in drag, and you've got Abbie.