The RNA properties of the Coronavirus are the same exact thing. Just happens to be much more deadly then the cousins.
Reinfection means being completely free of the virus after your body mounts a response against it and then it coming back and reinfecting you. Antibodies are a sign that your immune system mounted a response against the virus.
I'm not going to overstep by getting into pathology, because my understanding is limited in that area. I do know that the reports I've read suggest anomalies not found in the cousins. The virus is being found in numerous body parts never seen before with this type of virus (eg. testes). It is also confounding doctors in the ways it attacks the body. It seems to be attack different weakness in different bodies, again, in ways not common to other coronaviruses. I'm not a professional or an expert, but when I'm presented with a coronavirus that is different than any other in so many different ways, I'm not about to assume it's the same in any way without meticulously detailed verified research.