The relationship is probably to be good to the niece
My ancestors
1st cousin 2x removed
you will be his great great grand son/daughter..:)
First cousins twice removed.
Your brother's father-in-law is not related to you unless you and your brother married sisters from the same family. Then he would be your father-in-law, too.
There is no word in the English language for a relationship between grandmothers of related grandchildren
If you and someone else have grandmothers who were First Cousins, the parents of those grandmothers were siblings and you and someone else have great-great-grandparents in common. Therefore you and someone else are Third Cousins.
Your great grandfather's brother is your great great uncle.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
Your great grandmother's brother is your great great uncle. Your parent and the grandson of your great great uncle are Second Cousins. Therefore you and the grandson of your great great uncle are Second Cousins, once removed.
Your great grandfather's brother is your great great uncle.
your great uncle your grandmother's brother is your parent's uncle.... and confusingly enough when your parent is related to someone of the generation older than them (their uncle) it becomes your (great uncle) great relative.