Husband's ex doesn't want daughter knowing I exist

My husband has an 8yr old daughter who lives in another state. He gets to talk to her on the phone every week for about 10 minutes if he is lucky. Unfortunately our finances won't let him fly to see her and her mother wont let her come here. I am very happy that he gets to talk to her but her mother doesn't want the daughter knowing that he has a wife. I've been with my hubby for going on 2 years and 1 year married. What is some advice on how to bring it up? Or am I supposed to just let it be? My husband wanted to tell his daughter about me when we first started dating. The mother's excuse is that she doesn't want the daughter introduced to new women if we aren't going to last. I'm like hello we are married! He told the mother that it is on her when the daughter finds out and thinks her dad has been lying to her. We are afraid the mother will stop him from letting her talk to her dad if he tells her about me.