Adult Relationships

Okay this is Sooooo serious to me. I've discussed it with friends and now I want other opinions.

My husband was married previously for 7 yrs. The ex wife has an 11 yr old daughter from a previous relationship that my husband considers as his child. Hes been around the little girl since she was 2 and she calls him Daddy. She knows now he's not her real father

Here's the ugly: whenever we come home and he picks his kids up, he gets her too. He still semi assumes responsibility for her and I have told him to let it go! To me that's a responsibility he took on from ANOTHER marriage which is over. Her biological dad lives in the same neighborhood and I think they should all sit down and talk to her...she's old enough to understand. I'm not saying he can't maintain a relationship with her, but all the things he did, are no longer his responsibility because he's is now in a new marriage. I will never be obligated to have her in our home.

WHY Do i feel this way?

The ex wife has been terrible to me since DAY ONE! She's made up rumors, mad our relationship hell, made up fake Facebook accounts about me, repeatedly messaging me with lies about him, posting pictures of him on her Instagram as her crush and still claiming him as her husband...keeping his last name. Am I wrong?????