why not teach him to musturbate?

I think it was just a way to introduce a stranger into the household. If the stranger wasn't there then there wouldn't have been any corruption and therefore no movie.

My theory is that they really do have a missing brother. I think that either he ran away because he became interested in girls or the parents killed him because he tried to escape, so they decided to provide the boy they have left with sex to sate him so he won't do the same thing.