We had dobermans growing up. All great dogs but none of them passed from old age. Our experience was that dobies could possibly have an above average amount of health issues. All were smart loyal and extremely social animals.
Since I have been on my own we have had Rotties. Three in the house for several years. Awesome dogs. Our dogs were extremely social, laid back, and smart. Well bred "Bully" breeds should have a laid back temperament.
We have many friends and family members with Boxers. Awesome dogs. That being said, I have seen a fair amount of health issues with them. I would say slightly above the norm.
GSD. I would go this route for two main reasons. Timing and body type. In general, these dogs are athletically built and are working dogs for that reason. They certainly have many other fantastic traits. The timing that I am talking about is the fact that GSD's went through the same over breeding stages that many of the other bully/working breeds have gone through. I think that happened a fair amount of time ago and the breed has recovered well.
We will always have a Rottie in our house, as well as, some type of rescue dog as well.
Good luck to your sister. Dogs sure do improve the quality of life.
Oh yeah, get a female as well. Out of all of our dogs I would take any of the females to be at my side if **** ever got sketchy. Fearless.