Access to that facebook of someone who passed away?

[thebestok]

Very sorry to hear about her passing. I hope this can help.

With data protection laws using the forgotten password links wouldnt be recommended. I would suggest going through the correct method shown on the below link :

http://www.facebook.com/help/?faq=13941&...h=deceased

Theres a special form on this link for special requests which can be filled although the timescale isnt indicated.

As for yahoo, it would be classified as a separate organization so its dependent on the way your sister originally set up her account but most likely not integrated as one open ID as these have only been recently introduced.
I hope this is helpful.

I hope this helps you and my condolences in what is a very difficult time. my best wishes.