I am very sorry for you as I am in a similar situation but have a different role in the difficult menage.
My mother is 86 and has dementia but still harbours a desire for my sister and I to be friends, a hope I still, despite many years of provocation, share.
My sister is five and a half years younger than I am and we endured rather rocky relations as children as we had little in common and my sister was, as mum puts it, intent on 'goading' me'. This situation changed when she grew up and my DH and I spent a good, happy decade being the best of friends with her. All that changed when she got married and aligned herself (understandably, of course) with her new family.
My sister has, for years, been jealous of and resentful towards me for no real reason except imagined differences in our financial situations. We both have the same successful careers, are both happiliy married with lovely children and both have nice houses in nice areas. In short we have lives that most people would die for. Now I am retired and my husband has been made redundant her income is in fact four or five times that of ours with much lower living expenses yet still she regards me as 'rich' and herself poor. It is a narrative she has created and maintains against all evidence and sense and it allows her to be truly horrible to me.
My sister scapegoats me as responsible for all the ills of her job, situation and family. She sends horrible untrue and 'venting' emails when she is under pressure. She gets so furious when she sees me she cannot control herself or her dislike.
Thus, I am in the middle of a situation which, should my mother truly understand, would grieve her terribly. Yet I cannot help it as the position we are in is entirely of my sister's making.
I am stuck with keeping up a pretence of amity during Christmas and other visits for my mother's sake.
You did not explain the nature of the rift in your family, Floriatosca, but it might be that it is all down to the ire and resentment of one individual. If it is, then it would be terribly hard on the others to blame them.