I’m sorry for you loss.
My sister died when she was a few years younger than yours was, so I know a bit about it.
However, I also feel that you’re not the authority on grief, and if your sister was ill for a while as you say, maybe she encouraged her husband to start dating as soon as possible when she had gone.
To be frank whether or when he dates, marries, or fathers children is not really any of your business, and I think you might be projecting your anger over the loss you feel for your sister onto him..
She’s irreplaceable to you, but she may well have given him her blessing to move on with someone else as soon as possible.
I’ve observed that the times when one partner has a protracted illness and they die, the widower picks up his life very quickly and moves on, with her blessing.
I think you might cut him some slack and concentrate on grieving for your sister without throwing blame and anger onto anyone else.
I know life’s unfair, but it’s not your poor sister’s widower fault that she died, is it? He has a right to happiness, same as you now.
Again sorry for your trouble.
💐💐💐💐