This is a fictional scenario atm but it does happen to different people. Consider this a scenario that could happen to your sister or someone you know.
The scenario is, that the big brother tries to get into her room in night and she is worried he will try to do something with her. Now, she tells her small brother about this in hopes he could help her since this is a pretty sensitive situation. Things change and people are fucked up, anything can go wrong.
Now, the little brother who is old enough to handle this situation needs to find a way to fix this mess or at least control it.He has two choices:
A) Confront big brother and hear obvious lies and in the end start a massive fight.
Tell her sister to lock the doors and keep away from him and call him out if he tries to be inappropriate.Both the above solutions seem temporary.
So, if he:
A) Gets the Big brother married or tells him to go fuck some whore he might back off a bit.
There really isn't a diplomatic way of doing it.
Here is what I would do;
Analyse the situation, check body language and see everything in neutral. Remember people have different motives and when things go wrong, not even a blood related family would hesitate from killing you.
So, in keeping that in mind. Check everything and observe. Keep your presence at all times, just to show that you're watching everything. Be dominate and if it comes to the worse always think ahead.
Like you came in and he was trying to rape her. You will take him out or restrict him without creating a fuss. In short, you will need to seperate them and find more permanent alternatives.
I only want logical responses people. Please, no fuck everything join in the show ones.