Matthew is married to Olivia, and they have a daughter, Sophia. Olivia's brother, Lucas, is married to Emma, and they have a son named Mason. Emma's sister, Ava, is married to Ethan. How is Ava related to Sophia, and what is Matthew's relationship to Mason?

Provide a very brief response.
Ava, as the sister of Emma, is the aunt of Mason (Emma's son) by blood, and the sister-in-law of Lucas (Olivia's brother). Since Olivia is Sophia's mother, Lucas, being Olivia's brother, is Sophia's uncle. Consequently, Ava, as Lucas's wife, would be Sophia's aunt by marriage. Therefore, Ava is related to Sophia as her aunt by marriage. 

Matthew is the husband of Olivia, and father to Sophia. Mason is the son of Lucas and Emma. Since Lucas is the brother of Matthew's wife Olivia, Mason is Matthew's nephew by marriage. Therefore, Matthew is related to Mason as his uncle by marriage.