second cousin
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
If your great great grandmothers were sisters, your common ancestors are your great great great grandparents. That makes you 4th cousins.
Your grandmother's sisters are your Great Aunts.
Yes, along with your grandfathers' sisters, if any.
There is no relation between you and your grandmother's cousin-in-law's grandson.
third cousins
If she's not your mother, that would make her your Aunt.
go ahead if you want to marry your aunt
Sisters, mother, aunts, grandmothers, nieces, cousins, etc.
You are not related as you do not share a common ancestor.
no problem man! enjoy
cousin