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
Your brother's daughters are your nieces
Your brother's daughters are your nieces
Mary Lee Cantwell's husband was literary agent Robert Lescher. They had two daughters - Katherine and Margaret.
Grand-Nephew