I’m not naturally an empathetic sort, more a practical advice fellow, but my thoughts are with you and your family.
On a practical side.
Tell your siblings straight. There’s no easy way so best for all if you do it and more than that, get it out of the way so that the worry of that task doesn’t eat away at you.
Don’t worry about your 2yo. They are incredibly robust and s/he will deal with it far better than you. Unfortunately one of the teachers at my 5yo’s school died suddenly 2 weeks ago (chest infection that turned out to be aggressive cancer) and so the children have had some LEA ‘counsellors’ come in to talk to them and we as parents have learned a lot from that. there are many links that can support you in what and what not to say but from memory, the supposed best line is that when you die you don’t need your body anymore but the person goes on to live in heaven / the stars / other people’s memories (as applicable by your own beliefs and choices) From our point of view even though we are not religious we were happy to allow heaven and God to be used – it’s a CofE school, the teacher was a church goer and the staff and children were also supported by the local church. Hence it was easier to allow that myth to be used at this time rather than deal with a potentially distressed child AND at the same time tell her everything she was being told by the people she trusted at school was made up. When she’s older she’ll make up her own mind; maybe the same might be right for you at this time too.
On the what not to say line – one thing that stuck was that it’s not just like going to sleep because in some children that then creates real bedtime anxiety – what if I go to sleep one night and don’t wake up?
From your side – if you can’t deal with it by yourself then don’t feel you have to just because you are the eldest. See what help you can get, Macmillan have been superb for friends and my grandparents and they can give you the support you need so you can do what you have to.
Take it day by day, and enjoy what time you have left together. Best wishes.