What puzzles me is that Tanya would go with Sean (Stacey's brother), after finding out about the affair. I know Tanya fancies Sean, but clearly she'd hate Stacey after that. Wouldn't you just want to stay clear of the whole family?!
If it's for revenge on Max, then it's not really fair to the children is it?! They would have had enough to cope with, after finding out about Stacey. I think Tanya cares too much about her kids to put them through that.
Maybe Tanya cheats before everything comes out. But that would make her as bad as Max, wouldn't it?!