Still nursing? Do you mean on her momma cat? Or sucking on blankets or your sleeve, etc. A lot of cats do that for life. My Stormy did it until she died at 14. It's a security and comfort thing. It may be connected with being taken from their mom too soon. Stormy was found at 4 weeks old and I had to bottle feed her. So I know she had a reason to be a little odd so I just let her.
How old was she when she left her momma kitty to come live with you? I'm just curious because it's a theory of mine that it's related to leaving mom too soon based on my Stormy and others I have heard about that do that. But I would expect that if she does it now, she will probably always do it. Stormy did it with both me and my step daughter who she was also very close to. You're her mom now and she's doing it to feel content and secure. You can discourage her gently if it bothers you. Just make sure you still show her you love her after you pull your arm away from the nursing.