Sorry to hear about the gp passing away. The sister should not die just because she is alone, but she will be lonely so perhaps not eat as much as before when she had competition over food. Piggies are herd animals so they do need company.
There are a couple of options available. A neutered boar or a single baby sow, or even two. I say two, but only if the accommodation is large enough, because the sow is obviously five years old as well. There are lots of lonely pigs out there looking for a new home.
But it is always a difficult one. You don't want the one left behind upset, but also you don't want to upset her by introducing a new companion that she doesn't get on with, if you know what I mean.