Not 100% sure what she has, but a multi-child stroller might be helpful. I have a double because I had a little guy & a daughter that was 3 that was having issues w/ swelling from her JRA & was having a hard time walking. Plus we also had a 2 yr old... so the 2 & 3 yr olds would take turns in the one side w/ the baby in the other side - it was a huge help when I was out & about by myself w/ all 3 kids.
Other stuff would be anything she uses on a day to day basis... diapers, wipes, laundry stuff, bath supplies (for her or the babies), ect. Depending on how much bedding supplies she has - she may need a few more sheets or baby blankets also.
Frozen meals could also be a huge help - something she can basicly just turn on the oven & be done w/ it. Or if you live close, offer to bring dinner over once a week on say Wednesday (or which ever day is best for you) for a little while. That way she doesn't have to worry about it on that day - I know with a baby in the house & a 2 & 3 yr old sometimes it was hard to come up w/ the time to cook dinner... so I can imagen w/ 3 babies it will be even harder.
I wish you SIL luck!!! Also, congratz to the whole family on the blessing!!!