You are right.master_o_magic wrote::???: i dont see how mrs riddle could have had another child...she was killed in the riddle house in little hangleton along with mr riddle and their son 'tom' i.e lord voldemort....
But maybe things can happen along a very different line of thought? Do you remember Sirius's carpet showing all the family relations? We know for sure that LV is from an old family. We know, too, that the old families are related. So we don't need any hidden bros'n'sis because we might already know some people from his family. The Malfoys are part of his family. You bet. They may be from a different branch (no direct descendants from Slytherin), but still it may be running in the family...
I hope book 6 isn't one of those "blood is thicker than water" stories.