In my humble opinion, I would sign nothing for at least 4 to 6 mo. Take the car in to the best body shop you can find & also the Volvo dealership. Have the entire rear end checked on a hoist for hidden damage, get an estimate cost and time to repair. Also check rear wheel alignment on both sides. Your sister is entitled to have the car made whole. NOt new, just as it was. A buff job is just that.
I would also suggest that she have a doctor check her out. All that should be covered. I would strongly suggest that you sign nothing for at least 30 days. Make sure she is ok, and have the car returned before she signs off. Having had to deal with insurance adjustors the faster they can get you to sign off, the happier they are. Last time some clown hit my older Volvo, ('91) they wanted me to sign off that week. 6 mo. later, after I had had the car repaired, new rear bumper cover, both rear braces and bumper compression shocks, and a new rear hatch, tail light assembly, and paint job, we parted as friends. I signed off on the car after the work was done to my satisfaction, the meds, I waited for the full 6 mo. for. It was amazing how they went from "doing me a favor to settle for so much" to "and if there is anything else we can do...." I also suggested that if they continued to give me a ration of poop, I would turn it over to our attorney. Remember time is on your side, not theirs.

Sam