She's not Mo Slater, she's Mo Harris. It's true she's his mother in law. I'm not sure the dates of birth specified by Gnawer are official BBC dates, but according to the Eastenders website Charlie's wife Viv was the result of a fumbled sexual encounter that Mo had so we should assume she was very young (16/17?) at the time which would give a 10-12 year age gap between Charlie and Viv if DoB are correct.