My 'step dad' and mum are breaking up after 20 odd years together.
There is no point in lying about it - I never liked him. He is emotionally abusive to my mum, and has always despised me, my DH and kids, and made no secret of the fact.
There was a family row a few months ago - too long to go in to now, but he behaved shockingly - and it gave my mum (finally) the impetus she needed to end the relationship, although she hasn't moved out yet (financial issues).
In the months since, he has been making her life a misery, verbally abusive, telling er to get out of his house, she is a sponger, and saying despicable things about me and my family. It is really awful and I have egged my mum to come and stay with me in the interim, but she says she is staying put until the money issues have been sorted out. The house is his, btw, he was mortgage free before they moved in toegther and they aren't married, so she has no claim on any of it, even after 12 yrs of living toegther. But anyway...
I have just found out from a mutual friend that 'stepdad' has been having an affair for over a year, probably closer to 2 yrs, with a family 'friend', a woman my mum knows and has had over to the house etc. My source is reliable and I have evidence.
WTF do I do?? 
the relationship between shitbag stepdad and my mum is over, anyway, but he is blaming her to all mutual friends/relatives, saying it was her choice to 'throw everything away'....so he essentially gets away scott-free with his sordid little affair. It makes my blood boil!
WWYD?
Sorry this is so long!