First cousins
Your grandson is the great-nephew of your brother.
First cousins
He is your grandson. If his father is your son, he is your grandson.
If your daughter-in-law's grandson is a child from a marriage before she married your son, or after their marriage ended, her grandson is not related to you. If he is a child of the marriage between your son and your daughter-in-law, then he is your great grandson.
Nephew?!
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
Step-brothers
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
Your great aunt is the sister of your grandparent Her son's son is her grandson. Her grandson and you share a great grandparent and thus are Second Cousins to each other.
Jin is the son of Kazuya and Jun, and therefore Heiachi's grandson.
Your daughter's son is your grandson and you are his grandparent.