Hey OP,

NTA. Proud you stood up for your mom.

But... I'd also recommend staying off facebook, or blocking the family to prevent them from blowing up on you. Its easy to get pretty heated from horrible people, especially people meant to be family, shitting on those close to you. Maybe encourage other immediate family members to also go NC (no contact) on them. You don't need that toxic energy in your lives, especially after going through something crappy like this.

I've had my fair share of extended family members shitting on my terminally ill brother with cancer while he was alive. They refused to sit next to him, they spread this lie that somehow cancer catches on and we were all infected with cancer bacterium and so on. They pretended like they cared when he died by suddenly wailing really loudly outside the church during the funeral service. I say suddenly, because before his death they were especially nasty to him and our immediate family. One of them even stole something from his grave.

Whatever you do, don't fall to their level.