NTA for your specific question. Honestly, if daughter has been summarily disowned, it's a PERFECT TIME for bio-mom to take her in! But somebody or somebodies has seriously failed this girl for the last bunch of years. I'm already getting unsettling vibes from the description of her being a "difficult kid" with no real details or curiosity as to WHY. You and her DAD seem to have simply ignored her to focus on "your" family.
YOU are married to the VILLIAN in this story. And you're drinking his kool-aid. You can save yourself if you realize this.