DH and I got married when
SS was 4 (he's now 13).
BM got remarried EXACTLY 2 weeks later. It was a confusing time for SS as it was and BM didn't help by being a true PBFH........she hated my guts. So the best way I could describe it to SS was 'This doesn't mean that Mommy and Daddy don't love you, it means you have 2 more people to love and who love you as well!' He bought it and never had a problem with it, tho BM tried often to confuse the heck out of him about it.
Then when BM went thru her 2nd divorce, she suddenly developed some compassion. No increase in the smarts dept., but at least I'm not the wicked stepmom anymore and she willingly talks to me. And she absolutely has to now! SS needs braces and I work for a university dental college, where the fees are half of private, plus I can get half that, since I'm an employee there. Without me, there's no way DH or she could afford braces for SS.
I believe part of the problem may be what is going on in the BM's world, past and present. There may be issues from her past that make dealing with this issue extremely difficult. Regardless of what makes her feel that way, I agree with all the other posters.....it's a no-win situation, you're damned if you do and you're damned if you don't. Follow your heart and do what you feel is right for your SK's....in the end, their feelings are the only ones who matter anyway, right?