Emma and Mason are the children of Jacob, who is Isabella's brother. This makes Emma and Mason the niece and nephew of Isabella. As for their relationship to Oliver, since Oliver is the son of Isabella, and Emma and Mason are Isabella's niece and nephew, Oliver is their cousin. Emma is Oliver's first cousin because her father, Jacob, is the brother of Oliver's mother, Isabella. Similarly, Mason is also Oliver's first cousin.
Jacob and Isabella are siblings. Isabella is married to Ethan and has a son named Oliver. Jacob is married to Sophia, and they have two children, a daughter named Emma and a son named Mason. How are Emma and Mason related to Isabella and Oliver, respectively?
Provide a very short response.