If Louise is having a hard time "adjusting" to Susan's arm, she needs to get over it ASAP. If she can't get over it by herself she needs to see a professional to help her get over it.
Susan is not a random college friend of Dave's, she's his sister. There will be many times that Louise will see Susan over the next 40 years or so. If there is a genetic component to Susan's arm, there is a possibility that Dave and Louise could have a child with the same issue. What the heck is Louise gonna do then, give her child up for adoption?
I know many people can be uncomfortable when confronted with someone who looks "different" than other people. However, I would think that Louise has seen Susan enough to get over her discomfort, is there something else going on between them?