Dang it... I wrote like this mega long response but the internet monster must have eaten it. Ok, long story short. My cousin reacted the same way as your sis when his twin died. He turned on the family and partied HARD with the friends. I think your sis is running from the pain. She's probably not accepted it yet... or maybe she has. Either way it's obvious she's devastated. From the experience with my cousin, the best advise I can offer is to let her do her thing but remind her that you love her and if she needs to talk or a hug or anything, you're there in a flash. Other than that, I wouldn't bother cause it's only hurting you and you need to be able to grieve too.

Ry