Could be a half sister. Shrek's dad committed suicide when he was around 5-6. I've only seen his mother a couple of times, but don't recall if she was married, or had maybe remarried and been widowed (or divorced). All this to say that I've never seen the mother pictured with a man. But then again, the pictures I've seen of her were very casual, so a husband could have just been out of camera range.