How to handle daughter and mother in law who dislike you being with their father/son?

My future step daughter does not want me to speak to her, just to say hello. Her grandmother has been also feeding her information about me which makes it worse.

After I stuck up for myself I have been met with hateful treatment. I have had a few times where I’ve gotten upset and yelled at her and since then she says she does not want me with them, everywhere we go there is an issue with her behavior.

I have tried multiple things to help her with her happiness, and sometimes things are wonderful with us. She keeps going back to disliking me when I tell her not to call her younger brother names or hit him for example.

If I leave, she wins, and I lose.  I am so in love with my fiancé and his young son who has become so close to me (and me to him!!) as their mother sees them about once a week. However this situation is feeling sickening and toxic and I can not breathe most of the time!! Thank you!