I have to agree with Sion. There isn't anything to discuss here.
If you read the chapter and pay attention while doing so, you can't miss the "he killed his girlfriend" part.
And since all he did was kill the whole clan, it's quite obvious that she was part of the clan.
And everyone can come up with the whole inbreading/incest thing themselfes while reading the manga. It just makes sense that most of them did that (there are always exceptions) to keep the Uchiha blood strong and pure.
Nothing bad about it, since the clan was so huge.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"