Your great aunt's daughter or son is her grandchild. Your great
aunt's grandchild is your second cousin.
A nephew is a cousin to his aunt's son.
You and your aunt's son are first cousins to each other.
Your son and your aunt's son are first cousins once removed to each other.
Your mother's father is your grandfather. His sisters are your great aunts. Your great aunt's son is your mother's first cousin. The child of your mother's first cousin is your second cousin.
Your great niece's son is your great great nephew.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your mother's aunt's husband's sister is your mother's aunts sister-in-law but is not related to you. Her son is not related to you either.
You are a great aunt or great uncle to you nephew's son
Second cousins once removed.
Your niece's son is your great-nephew. You are his great aunt or great uncle.
Your granddaughter's son is your great-grandson.
Your nephew's son is your great nephew.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your husband is great uncle to your niece's son (your great nephew).