Legally there is nothing to stop you as long as she is of legal age. You do not share a common ancestor.
The family may not take too kindly to it.
i don't see anything wrong with it.. but if you end up ending the relationship, it could get awkward!!
Second cousins.
You and your first cousin's son are first cousins, once removed.
Your cousins child is your first cousin once removed.
If it is not your common grandparent, there is no relationship.
Your uncle's first cousins are also your parent's first cousins and so are your first cousins once removed. Similarly his second cousins are your second cousins once removed.
If two people are first cousins, their children are second cousins. If two people are fifth cousins, their children are sixth cousins.
Your children and your first cousin's children are second cousins.
second cousins to his cousin's children, you'd be third cousins
If I were you I would call her by her name, if she has a nickname she likes to be called then that will do, just do your best to keep it casual. As a practical matter, the English language does not have a relationship term for a stepdaughter who is not your stepdaughter any more because of a divorce. If you have a continuing relationship with her, you should feel free to continue to call her your stepdaughter, or even your daughter, depending on how close the relationship is.
You are First Cousins
Well technicaly it is her godmother!!