Answer:
   
   There are many believe that Judaism teaches that the family must
   consider as dead (and as a result, perform appropriate mourning
   practices such as sitting "shiva") for a child who marries a non-Jew.
   However, it is not clear the anyone does this. It is definitely not
   halacha (Jewish law), nor is it widespread enough to be a custom.
   
   This "legend" arose because, until recently, those who had interfaith
   marriages often abandoned Judaism, becoming apostate Jews. The custom
   of sitting shiva for apostates seems to be based on a misunderstanding
   of a passage in the Or Zarua (13th cent), which stated that Rabbenu
   Gershom (11th cent) sat shiva for his son, who had become a Christian.
   My understanding is that Rabbenu Gershom sat shiva when his son died,
   despite the fact that he had apostasized, not when the son became a
   Christian. The halakhic discussion of this point, which starts in the
   Or Zarua, goes back and forth on whether or not we follow that
   practice, but, there is no suggestion that we should sit shiva when
   someone leaves Judaism.