This is confusing. When you say "my children", are you referring to kids you have with her dad? If so, are you married to him? I ask because you never call him a husband. This matters.
If he's a bf, she's right. Even if they're technically half sibs, she doesn't have to acknowledge you as her "stepmom". If he's your husband,this is all different.
EDIT: Yikes. What does your husband say in all this? These are his kids and his grandkids! (I think). I have no idea what to suggest, unless this idiot daughter of his still relies on him financially or in any other way. He needs to cut that off and tell her if she doesn't recognize the importance of family, fine...but it's a 2 way street.
Also, I don't think you were wrong to tell your kids the truth if you were asked directly. If you did this one your own, no biggie, but it might have been better to wait a bit. They must be pretty young, and it wouldn't be cool for them to get caught in the crossfire.
This is really weird!