Omg. Your sister is insane. I had a cousin who passed under similar circumstances. What is ironic is that my brother, who failed to mention his own wife's passing for six months, was the one who immediately blabbed the cousin's sordid details to me. Like it was his effing job. The rest of the family (the deceased's SIL who initially notified me) was much more circumspect.

I hope you can go on this journey and embrace the comfort of your remaining relatives as you deserve.