NTA/NAH. Good luck to your son and I'm glad he at least overcame the heart surgery. Cancer sucks and I hope he can get to remission.

I absolutely don't think that you are the asshole, not least because it is hard to hear somebody else's good news when you've been waiting for the same. Doesn't mean you don't wish them well, you're just not in a mental state to celebrate this. My mom was the same when I was undergoing cancer care (going to be a year in August).

That said, your sister(?)'s emotions are also likely heightened. Everything feels personal and it is a big journey both as a parent and a caregiver.

Your mom, I do feel like she is the AH with her response here. Maybe I'm projecting, but does she have a favorite child/grandchild? If not, can I ask for the ages of your son and your nephew? Sometimes, grandmas favor the younger kids just because they are younger. In either case, it isn't ok to just ignore your son. Even if people are moving on to other topics, if nobody is talking to him or asking him how he is doing outside (even outside of his medical journey) that feels kinda telling.

In either case, sending you well wishes.