Son-Sister of Helm?

Rereading Return of the King in preparation for the Big Day, I just found
something really puzzling: at the funerals of Theoden, the names of all the
lords of the Riddermak are told, and we find a "Fréalaf (that's the name
he's given in the french translation), Son-Sister of Helm". I know that it's
quite possible to be a daughter-sister (Chinatown, anyone?), but a
son-sister? Can someone who has read every piece Tolkien has ever written
about Rohan and the Rohirrim explain that one?

Gérard Morvan