To be clear to anyone reading this thread who may be a SM and not recognise themselves in these stories, I know perfectly well that loving and healthy step-parent relationships are perfectly possible. My brother has a blended family with one DC on her side and two on his, and while there are frictions like you'd get in any family the affection of both parents for their step-DC is genuine and unforced and all the DC are thriving. But as the adult child of a profoundly toxic step-parent relationship I'm really glad to have found somewhere to vent.
I could tell such stories. The time I turned up at short notice to drop off Christmas presents with DH (then just boyfriend) to be met by SM screeching at me from the front door about how I was wicked and evil and wanted her to die. DH had convinced me to turn up, because 'how could that be bad, that's what families do' and when she reacted like that he was 

and finally understood what I was up against.
Or when they paid for all of her son's very expensive wedding but none of mine. And only grudgingly turned up. Not that I needed or even wanted the money - we're adults and solvent - but the favouritism still galls.
Or the spurious reasons she has invented for the last four Christmases in a row in order to justify refusing to allow my dad to come and meet us for his annual furlough from captivity. Basically the deal is that I have done something appalling to offend her (the nature of the offence changes every year and having had no contact with her for the last year is no barrier to my committing new offences) and if I want to see Dad, I must first go up to their house alone and submit to several hours' character assassination by her, aka 'clearing the air' before we can all be friends again. The fact that I refuse to do this usually constitutes next year's offence.
I'm pregnant now and a fully expecting that Dad will have little or no contact with his new DGC - the first in the family for a decade. He won't be allowed to. My mum has also remarried and I expect that my child will grow up knowing DSF as the only grandfather he had, with my dad an odd, awkward old man he saw maybe twice in his life 
Phew. That feels better. My dilemma is this: I'm practically NC with them, basically because I refuse to play her game as she is batshit and hates me and it's a waste of energy. So do I call and challenge my dad to put his foot down, for the sake of having a relationship with his grandchild? Or do I just give up now?