Welcome. I'm so sorry to hear of the complications your sister in law is facing. What a lucky family to have you reaching out for support and considering what is most helpful for them.
It is not uncommon to continue to face complications after birth. Pre-eclampsia and HELLP can leave long lasting complications, but each person is very unique. It sounds like she is in good hands- and I will keep good thoughts that her condition improves with time. I'm sure others with HELLP experience will also chime in.
In regards to what is helpful letting them know you are thinking of them and offering to be at the hospital with either their child or wife (so that someone is always looked after) can be helpful. Everything at home can really fall apart when you spend so much time at the hospital- so feeding dad- keeping the fridge full of healthy easy/on the go food, offering to keep track of mail/bills etc, mowing lawn. And depending on your relationship, these might be things you just do so dad doesn't have to worry about it.
If they are far from the hospital and dad will have to make lots of back and forth trips, gas gift cards- sometimes you can also provide "gift cards" to the hospital cafeteria too.
And don't forget to say congratulations- while it's not the way anyone wanted this lil' one to come into the world- lil' one still had a birthday and they still have the right to be proud new parents.
Keep posted as you're able,