I havent read all the other replies so I dont know if this has been mentioned but calling you mommy can cause him to lose custody.
You are not the mommy.
Yes, you can stop the kids from calling you mommy by saying "no, i am not your mommy, you have a mommy and she loves you very much" even if you dont believe it, or think it. do it for the kids you love so much.
it is not a good thing to have other peoples kids call you mommy or daddy.
this can cause the custodial parent a lot of grief if the non custodial parent decides they want custody. seriously, it can.
the father of your stepchildren needs to nip that in the bud, quickly.
as well as taking away presents given by the MOM(not bio mom there is no biomom its just mom)
It's not something I will tell them not to say, it's their choice and the courts cannot take away their right to say what they want to say.
Are you telling me that if they called some stranger mommy, she could have them taken?? Thats silly and I doubt it.