I think many fathers take on their new partners kids, cook, clean, look after them, drive them take them to school pick up etc because they are there permanently and that is what families do.
Then his kids come over and the SM, in many scenarios, will not do the same as the above - you can see where misunderstandings happen.
If you make an effort - great I admire you, but some of you speak of your DSCs in such a derogatory way I find it quite sad.
Then you can be my DCS SM, just make it so difficult for them to come round, to their DFs house, refuse to have them on holiday, that they have not had an ON for 1 yr, because she wants time with their Dad on her own. Ignore them/ disengage when they do go round, not speak to them and make them feel unwelcome.
In my scenario - I am their mother and will always look after them, but what in essence I also am is their free childcare, babysitter - because I do his share aswell. I pay through the nose for childcare. Going to the gym after work, is not cheap as I factor in the £7per hour child care I need, going for a child free meal with friends is £20 before I go out - ergo I do not go out often!
Not sure it is always the SM who sucks it up