Hi
This is a hard one....
I would talk to your friend and see if she talks to her sister about it to see how she feels about everything.
Or even ring b4 you visit and give her prior notice that your going over then she can leave (hide in bed room) if she don't want to see you or your friend might say she might come over to see you instead.
Also you could send her a sympathy card letting her know your there for her.

Cheers
Chris