If Theresa's daughter is my daughters mother then who am I to Theresa?

You are either a son in law or a daughter in law to Theresa. That's because her daughter is the mother of your daughter. That means you're married to Theresa's daughter, who is your daughter's mother. It is a common misconception that Theresa is actually your wife. Of course we don't have information whether they're all connected biologically or someone might have been adopted.