He is your grandson by marriage or grandson in-law.
You call your daughter's baby, your grand-duaghter.
would it be a brother in law? not sure about this one! Is it your husband/wifes daughters brother?
would it be a brother in law? not sure about this one! Is it your husband/wifes daughters brother?
Your niece's daughter is your grand niece, or, as some would say, your great niece.
Your daughters husband is you son-in-law.
Zara Phillips - she is the daughter of Princess Anne and her first husband, Mark Phillips.
Uncle
Mary Lee Cantwell's husband was literary agent Robert Lescher. They had two daughters - Katherine and Margaret.
Your brother's daughters are your nieces
Your brother's daughters are your nieces
Grand-Nephew