My brother is in college, and stays with my family about 25% of the time. We have our basic plan that if anything needs investigating I (and only I) will go out and investigate. Both wife brother and child are to stay in their rooms and depending on the situation call 911. If any shots are fired they are to call 911 (if they haven't already) and listen for me to yell our (previously and mutually agreed upon) phrase. If they don't hear it, they are to turn the hall lights on, and shoot any hostile who comes down that hallway whether they have me (somehow) hostage or not. Now there are a variety of things that could possibly go wrong with this, however simple instructions I've found are the best thing for my wife.
In your situation, I would have my brother stay up stairs, with out his guns. It just seems that would be "safer". You said money was tight, and trust me I understand that. I would see if your dad would be willing to have your brother's (or technically your dad's?) shotgun by his bed. Then establish responses to what ever scenarios you want.
Just my opinion, so take what you deem useful out of it, and toss the rest.
