I have grown up in a step family and unfortunately had a particularly unpleasant stepmother. She was very jealous of the relationship I had with my father and eventually drove me out of the family. I vowed to never be that person if was ever to be a stepmother. 
I have been with my partner for nearly four years. We moved in together just over two years ago. I have a son from my previous marriage and he has two from his. 
His two children have a mother who enjoys playing the victim, is manipulative and exceptionally greedy when it comes to money. This has rubbed off on to the children and they are very vocal about their dissatisfactions on a regular basis. I have tried hard to make things fair when it comes to dealing with them and my son. Sometimes I feel that I am punishing my son unfairly to show my step children that I am playing fair. I do not discipline my stepchildren any different to their father and generally keep a distance from the relationship between my stepchildren and their father, as they are very needy. However....... it's not right... it never is. They complain to their Mum that I'm unfair and harsh. I am so fed up of it. I just don't know what else to do. I am thinking of staying away when they come to stay, but my son adores them. Any advice would be most welcome.