Oh God, I am so sorry to hear this ginger. Old threads mean nothing when it is happening to you.
There are so many things you could help your sister and her children do, but really I think the most important thing is for her to spend time with everybody she wants/needs to spend time with. If she feels up to it, your sister could try and write letters to her children. Although this I think is a nearly impossible ask for someone coming to terms with their own death. I know my DH tried to do this for our DDs, but by the time he was emotionally ready to do it, it was too late for him to do it physically. Same with plaster casts of hand prints. Really nice ideas, but only if your sister is up to it.
Whereabouts is your sister? Is she at home, in a hospital, in a hospice? Please see if she is happy where she is (sorry if this sounds obvious) and happy for it to be where her life ends. Many people want to die at home, but because of costs and bureaucracy so few are actually given that wish. Do you have enough practical help so that you are able to spend as much quality time (sorry for the naff term) with your sister? If not, please push for it as much as you can. Macmillan are usually fantastic at helping with this.
Wrt to the children, I would say be age appropriate but be open too. When DH died, I was holding his hand and chatting to DD1 (who was 3.5 years at the time). He died peacefully and it was so natural that DD1 was there. I guess children look at things in a very black and white way and will only be scared of something if that is what they are told they should be. I also think crying in front of them is not a bad thing to do. It's a big fucking thing happening and tears are a very natural response to that. Your grief (and theirs) should not be a dark, secretive thing.
There are loads of organisations to help following death. Cruse is one - and you may well want to have bereavement counselling through them. For children, Winston's Wish is fabulous.
A warning, with death comes a load of paperwork (at a time when you least need to be dealing with that stuff). There are so many different organisations who need to be informed, all of which demand an original copy of the death certificate - so get several. I am actually working with my MP to try and make this better as it seems a ridiculous thing to have to do.
Be very, very kind to yourself. It is a tragic thing happening and you need to look after yourself as much as you can. I hope you have good friends and family around, supporting you through this time. I really hope I haven't upset or offended you with anything I have said - some of it seems so harsh and matter of fact. But I promise, I do know how bloody hard it is.
Life really is truly shit sometimes. My thoughts and prayers are with you, your sister and her family x