What am I to my nephew's son?

His great-aunt or great-uncle, the same as for a niece's child.

What am I to my niece's grandchild?

Their great-great-aunt or great-great-uncle (great-grandaunt/granduncle).

You are your niece's child's great-aunt or great-uncle. They are your great-niece or great-nephew.

You are your niece's child's great-aunt or great-uncle. They are your great-niece or great-nephew.

12.5% shared DNA on average. Real amounts vary.

you them common ancestors double line: marriage

You are your niece's child's great-aunt or great-uncle (grandaunt or granduncle in genealogy). They are your great-niece or great-nephew.

“Great-great-aunt” is a common mistake here. The extra “great” starts one generation further down, with your niece's grandchild.

How we worked it out

Follow the chain one step at a time:

Your parents are their great-grandparents: they are your common ancestors.

They are 2 generations below you.

…

Tap relatives in order, like mother, cousin, daughter

More: grandchildren, great-, half-, step- and in-laws

Any sex Grandchildren and great- Half Step In-laws

Or type it the way you'd say it

…is what to

I am