I'm going to tell my sister that she'll regret going back to work
But it's just a job. Why do you think she will regret it?
I think she's a bit too tired because of a premature birth of a baby.
It is okay to tell your siblings what you really think especially if you are concerned about something that will affect them. It will help them to have that information when they make their decision.
She needs to take a big break or she'll get stressed.
You are such a caring sibling, that's great you look out for her so much and try to tell her these things! She definitely needs all the rest she can get, such a traumatic experience can make her feel even worse at work. I hope you can further help cheer her up!