You would be their great Aunt.
Your grandfathers' sisters' grandchildren are your second cousins.
You are a great-aunt.
No, they`d be the second sisters great nieces and nephews
Your son's children are your grandchildren.
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
Children of sisters are first cousins. Grandchildren of sisters are second cousins. Great grandshildren of sisters are third cousins.
Yes, he is their great uncle.
You can say "los nietos". This includes male and female grandchildren. If you are talking about exclusively female grandchildren, you can use "las nietas".
cousin
brother-in-law
Great neice
Your grandmother's sister's grandchildren are your second cousins.