What if you show a kindness that they did not show to you? That you extend some flower? ....share your feelings with your mother and brother about what was and what is. That you've mourned having family and a mother. You can't change the past and their actions had an impact to make you never want to align with an unloving organization and that their being left to fend for themselves without support is like how you were treated. You could send a token amount of money $50-100 or whatever you are comfortable with .... in memory of the mom you once knew, once loved, and mourned. You can share that you found out about her efforts to keep you isolated. In her warped hopes that you'd return to the organization because you missed family.
Their desperate outreach is evidence you can use that they 1) still consider you family and 2) have no support from their so-called friends or organization, despite all that they have done to be "good witnesses." I feel pity for them as they no doubt are pondering so much now in their desperate state. Christ was forgiving and not seeking revenge. At his death, he prayed for his enemies for they "knew not what they do".
Your success is your greatest revenge. Your mercy and extending some flower of peace would be another kind of success. Yours, of course, to choose. As a mother myself, I can't imagine cutting off my sons (and she did keep some communication, right?) and never seeing them again. But also can see one pressured by the peers she kept to be diligent about shunning to try hard to bring you back into compliance.
Forever cutting off or peace gift and message on her last journey to death and possible great remorse - it is your choice this time. I feel for you on the difficult journey you've traveled and hope that she could be moved to open her heart to regret and forgiveness. Sadly, I realize this may not happen and know of others in my family still suffering from distanced parental relations even though they were never baptized. Wishing you strength and peace in whatever decision you choose.