Ex is threatening that our child will hate me if I have more kids

My daughter is 5. She’s been though a lot due to her mother. I have full custody of her but her mother is saying she’s going to take me to court soon since she’s now out of rehab. (Earlier this year she overdosed)At our last visit with her. She out of the blue asked if I was dating anyone (I’m not) and then went on to say that if I had more kids out daughter would be resentful & want to live with her full time because I wouldn’t have time for her.Now I’m an only child. So I really don’t know about siblings but this isn’t true right? Kids adjust to new siblings right?It’s gotten me nervous that what if she does decide she’d rather be with her mom if I marry or have more kids. I’m having a lot of what ifs. I love my daughter and will anything to keep her happy. I had been thinking a step mom or new sister might be something she’d enjoy but her mother put a worry in me.I’m not even interested in anyone at the moment so this would be years down the line.Thanks for any advice. via /r/Parenting https://ift.tt/2raaLE7