There are Minhagim that a parent sit Shiva and say Kaddish if a child marries a gentile. What is the proper thing for a child to do when his or her PARENT marries a gentile. (The other parent is the primary care giver).
The custom to sit Shiva for a child who marries out is mentioned in the Kitzur Shulchan Aruch, but rejected by many authorities, and is not common custom. It is not the custom when a parent marries out, so certainly when it was not done originally, there is no cause to sit Shiva.