I'm uncomfortable that my girlfriend (24f) is allowing her housemates brother to stay at their place while her housemate is out of the country
What makes you worry about her taking care of her friend?
I'm worried about the guy, because it's so creepy and I can't trust him
It's understandable to want to protect your girlfriend. If you have these feelings about this person you should communicate that with your girlfriend..
I'm just going to keep my distance, I don't know what he's doing there.
Keeping your distance may be best to cause no issues down the line. As I said before, it would be beneficial to communicate this with your girlfriend. That way boundaries can be set if this person is off-putting.