If you are married to your daughter's mother, then her boy is your stepson. If you are not married, then he is no relation to you.
Either way, he is your daughter's half brother.
Your mother's son is you or your brother. Your mother's son's daughter is your daughter, or your niece. Your mother's son's daughter's mother is your wife, or your sister-in-law.
He is your first cousin twice removed.
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
Physically it is possible, just as mother and son can have sexual relationship.
Your daughter's grandfather's brother's son is her mother's or father's first cousin and her first cousin, once removed.
B is A's daughter. A is B's father.
in western cultures it is the same for all the children's children. grandmother to grandchild.
She is the co-mother in law of you and your daughter's father in law is your co father in law. :-)
Your daughter's daughter's son is your great grandson, and you are his great grandfather or great grandmother.
Your brother's son or daughter is your nephew or niece respectively.
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Your sister's son and your daughter are first cousins to each other.