BOTH. Son, you never EVER violate your stepsister or any other girl/woman like that! It’s extremely disrespectful, a severe invasion of privacy, and it’s literally ******* creepy. It’s entirely inappropriate and you should never, ever do it again. Now hand over your X-Box until the lesson sinks in. Daughter, in situations like this, you come to an adult to talk it out and WE will determine the punishment! Throwing someone down the stairs is completely unacceptable with the exception of if you are in immediate extreme danger and need to protect yourself. You could have severely injured or killed him! Does that seem like a proportional reaction?! Now hand over your phone while you think that over.

Okay both are in the wrong (mostly the creepy ass stepson...). The son is acting like a creep sniffing about the sister's underwear but to try to end his life is a REAL excessive way to combat it. They need to sit down and have a serious talk with an adult before one becomes a sexual deviant and the other becomes a murderer. Act fast ASAP!

Teach your step son to be respectful of her room and privacy. He is being a creep going through her underwear. What’s next spying on her while she is in the shower ?

I have to ask how old the boy is? I mean if he is 2 or 3, he was probably just curious and opened drawers. If he is any older than 5 though, that's a bit weird. He needs to be taught to respect others privacy and belongings but she also can't throw someone down the stairs and potentially injure him or worse. I can only imagine what she would have done had he been reading her diary or something. Shove him in the oven and cook him? Yikes!

well he shouldn't be in her room without her permission and shouldn't be in her underwear drawer forsure so I would say stick to her side. she's fine for what she did. just tell them both to never do what they did ever again.

FML you're referring to which child in the title?

Not sure how old either kid is but assuming the boy is at least a teen you can tell them both that their behaviour is unacceptable and put a decent lock on your daughter's bedroom door to protect her. also check for cameras in the bathroom etc.

Both. The approach and depth depend on their respective ages

There's a few things people are missing in their comments age is the biggest determining Factor here. If the boy is pre-puberty or only starting puberty this is most likely just natural curiosity any male that had a mom or a sister has looked at their unmentionables at least once in their life when they were very young. So you need to try to figure out why he did it ,was it natural curiosity, was he using them for masturbation, or is he just realizing that he may be of a different sexuality and he may have wanted to try them on and see what they looked like on himself. Also your daughter's violent reaction to the situation is also a major red flag in her own possible future behavior. Your children need to be talked to separately and then together to solve the situation with proper communication before you end up with a funeral of one child and prison of the other. You also might want to consider getting them both some therapy and putting locks on both of their doors, but make sure you keep your own key in case you need to get in there. and I hope you understand this is this is coming from personal experience I've raised half a dozen kids and had a very similar situation many years ago.

What your daughter did might not be okay but if it is a repeated offence then stepson deserves it. Protect your daughter. She will need it if your stepson is like that.