Vilkas loses control while he's a werewolf and ends up raping a not transformed Farkas. The thing about werewolves, though, is that just like wolves, they knot. Vilkas regains control of himself while he's knotted inside Farkas and is horrified by what he's doing, but he can't even pull out until the knot has gone down. Angst, angst and angst for Vilkas, made even worse by Farkas being shaken afterwards but still incredibly understanding and adoring his brother as much as ever.