Just something that's come up on other threads:
For me, i wouldn't
sign school books
presume to go to any school function
cut my SCs hair
Organise any activities which would overlap time at their mums
Go into DH's ex house unless it was an absolute dire emergency
i'm sure there are others.. but where do you feel is drawing the line?
And for mums - what does the step mum do that gets to you (no, breathing isn't an acceptable answer