Can My Mother Publicly Post My Deceased Brother's Private Texts?

My brother(36M) overdosed on February 7th. He was estranged from my insane mother, but since he did not have a will she became automatic next of kin and executor.

My mother is refusing to do anything she's supposed to, contact beneficiaries, handle his bank accounts (she says it's "not her problem" even though...this is her literal job), and worst of all, all she cared about was getting his personal possessions, mainly, his phone. (She has a vendetta against the rest of my family, long story, she is mentally unwell.)

She has hacked my brothers facebook and started to post slander about my family (as well as on her facebook), However, she just posted a private text between my uncle and my brother before he died (my brother was in a tough spot near the end, part of my family wanted to know if he stole money from my grandmother. He never did and never would). My mother posted the text of my uncle asking if he stole money and is saying that this "pushed my brother over the edge and made him kill himself."

Can my uncle sue? Is this against the law? Do I have any rights as the next, next of kin to challenge her misuse of power?