I think we all have a sneaking suspicion what Jr. was up to.
Okay, what was he up to?
The "bad-guy" was still outside of the females house
The good guy (step father and brother) was asked by the neighbor sister to investigate a possible burglar (prowler). Being a good brother and taking care of family, he did so. So he was 100% in the right for what he was doing there. He found the prowler who is our bad guy stepson. When that happened, the all black and ski-masked bad guy came at the good guy with a knife. There was a scuffle. The bad guy was killed.
ALL of the legal aspects for self defense are covered. It does not matter that the bad guy was outside the female's house because he posed a threat to the good guy.
As for reports of a masked prowler in the neighborhood recently, nothing shows up in the local newspaper's police blotter.