I was married and we had a daughter. We divorced. My ex-husband remarried and has a son. Our daughter and his son are half-siblings - share a dad. What am I to his son? What is his new wife to my daughter - stepmother I think?
Stepmother is a woman:
who is married to one’s father after the divorce of one’s parents or the death of one’s mother.
The relationship is established only when you are (legally) married to a man who had a child or children with his ex-wife or deceased wife. You would be his/her would-be stepmother if you are planning to marry him.
You are just an ex-wife or friend (if you maintain friendship with him) of his father. There is no legal relationship between you and him. The same applies to the relationship between your daughter and your ex-husband's new wife.
[Oxford Online Dictionary]