Best just to get on with it.
I had a situation similar before, and it annoyed me to no end. In the end i just thought 'Right, F*ck ya, Your loss...' and moved on. Definately the best way.
I was kind of seeing a girl, pretty much the same situation as you, and i had become good friends with her brother through spending time with her. Anyway her parents went away for a weekend. She invited me to stay for a weekend, and invited a couple of my mates aswell. I think she felt that i'd be more comfortbale with them there or something, but anyway. When i got there things were fine. We were getting on really well, and were all over each other. Then half way through the night she got up and stormed out into the garden and sat out there. I asked her brother what was wrong, and he didn't know. I went and asked her, and she just wouldn't tell me. I thought oh well, she'll be fine later and had a laugh with my mates. Then about an hour later her brother told me she was upset with me, and sent me out. She said she was annoyed because i hadn't spent enough time with her, and that i was having a laugh with her brother. I apologised, mainly just to make things run smoothly for the night, and things were fine. We then slept together on the couch at the end of the night. Half way through the night she got up and stormed off for some reason. I followed her, asked what was wrong, and she just kept telling me to go away. So i spent like the next 3 hours weightlifting in the rain in her back garden. Was a good laugh in the end.
Basically, what i'm getting at, is sometimes you haven't done anything, and it just works out that way. Get on with it.