This is a brain teaser, a very tricky question. Here's the answer:
I am Teresa's son-in-law.
To have this question be answered in a manner that it will be understood easily, let us suppose that Mary is Teresa's daughter and my daughter is named Maria.
The question is: What am I to Teresa?
Teresa's daughter is Mary. Mary is Maria's mother who is also my daughter, then I am Mary's husband and Maria's father who is also Teresa's son-in-law.