I'm truly sorry for what has happened.
Many people are addressing how to communicate, but few are addressing what to communicate. At this time, your sister-in-law is tired, afraid, and a new mom. Her mental stamina is low and she is trying to heal. Making it harder, her potential to heal won't be apparent immediately, and can take several weeks or months to show.
Let her spend time with the baby. If things go badly this may be her crowning achievement, and if things go well, this may be her greatest mental uplift giving her the energy to heal.
Spend time listening as well as talking with her. Always give her comments to you priority over your comments to her. If her time is limited, there's much she will want to say - you have to let her get it out.
Just take things day by day. There will be good days and bad days. Bad days can actually be good news - healing is tiring, and while her brain swelling goes down and she recovers she will be extra tired. As parts of her brain switch back on and fumble to find their mental feet, she will sometimes seem off-balance.
Finally, given the affected area, modify your expectations of touch. Contact is important, but it should be somewhere she is connected to. If she can't move or feel her hands (which are two quite separate things) but she can move her eyelids, contact with her face might work better. Give her a say in that - she will guide you. Touch and intimacy are vital to her wellbeing.
I hope she makes a full recovery. She may well not. Take what you can get, listen to her, and do the best you can as a family to work with what you now have.
Congratulations on the new baby. I hope they will grow to know and enjoy their mother.