I agree that it doesn't feel good having to tell a child that their bio-parent is lying, but it is the right thing to do. I didn't tell my niece of all the stuff that we'd gone through to keep her..the really bad things her bio-dad and his parents put us through, until she was about 13. She was livid, but not at us.
She and I have our differences, argue at times, but the one thing she's learned about me is I will tell her the truth..don't ask me anything you don't really want an answer to
When her bio-dad was in the ER, his stepmom called and told my niece "he's DYING! You need to get here NOW..but come by yourself" - WTF? She knew I would find out his condition (used to be a nurse at the ER he worked at, knew how to get an answer without breaking privacy rules). He wasn't dying, he had OD'd but was fine.
I offered to take her to see him, if she wanted to go. She chose not to, but did tell me "if I did go, I'd want you there..we don't always get along, but I know YOU have my back and you wouldn't let anyone run all over me".
I think you are doing a fantabulous job with your stepdaughter. I know it's hard on you, but I'm really glad you're here for support for YOU! The only reason I have the relationship I have with my niece is because I learned from all the people here.
Hugs and prayers,
Amy