I have dealt with the deaths of both my parents and all of my grandparents, so I kind of know how to deal with it a bit, but it's not the same for everyone.

I remember when my dad was dying and my grandpa, they just wanted us to talk like we always did. Occasionally, they would talk about their impending deaths, but mostly, they wanted to make the most of the time they had left.

Avoid saying things like "it'll be ok and I know how you feel", even to your sister. I guess these things are ok with some people, but for me it felt incredibly invalidating, because nobody can "know" how we as individuals feel, and it won't be ok, not for a while at least. It will be a huge, heartbreaking part of life.

As for what is expected of you, if you are able to, offer help with cooking, cleaning, etc. If there's nothing specific, maybe just take care of the dishes so your sister can be with her husband, wipe off a table, take out the garbage. Little things that seem insignificant on a daily basis, but take her away from the time they have left together.