M.W.
I think it's wonderful of you to be concerned with helping her. Afer my first baby I found myself going through PPD issues & it was just awful.
As the other ladies have suggested, the best thing you could do for her is to let her have a break. And don't treat her as thought she were fragile; I'm certainly not assuming you would, but it might make her feel uncomfortable if you were to inadvertently behave as though she "had a problem." Make sense? Treat her normally, don't be too coddly & give her a nice break once in a while.
You're a great sister in law to help out!