Marcus has a sister named Thea. Thea is married to Adrian, and they have two children, Sofia and Luis. Marcus is not married but has a daughter named Isabel with his partner Ana. How is Isabel related to Thea and Luis, respectively, and what is Adrian's relationship to Isabel?

Make your response very short.
Isabel is Thea's niece because Thea is Marcus's sister, and Isabel is Marcus's daughter. In regards to Luis, Isabel is his first cousin because their respective parents, Marcus and Thea, are siblings. Adrian is Isabel's uncle by marriage, as he is married to Thea, who is Isabel's aunt by blood.