If teresa's daughter is my daughter's mother who am I to teresa

If you give everyone a name it's a lot easier. So we have Theresa. The person who is related to you somehow.

She has a daughter (let's call her Erica)

Erica is my daughter (Janet) 's mother.

Janet is my daughter. Erica is her mother.

Me and Erica are Janet's parents.

Theresa is Erica's mom. If me and Erica are married, she's my mother in law. If not we can give her the title of my daughter's grandmother.

SOO there you have it. She's my daughter's grandmother.

Teresa is your mother in law / you are Teresa's son/daughter in law