@Oela, are there any grand parents around? Your DH and BILs parents? Both your DD and X are their grandchildren, how would they take the news? Support X or your DD?
I agree where you said a lot of teenagers have issues whether they've been abused or neglected or what ever but they take it out in other ways.
You're such an amazing mum who has held it together so well. I really hope your DH comes around and is angry at X as you are. I agree with the PP, you need counselling together. I know it's okay not to see eye to eye on some things in a relationship, but this is something you need to be on the exact same page with, the same sentence in fact. For the sake of your DD and your marriage.
I hope you can enjoy your weekend. Hold your DD tight and give her a really happy and fun weekend. Maybe sit her down over some ice cream or a favourite treat and just ask her how she's feeling. See whether she brings it up