I believe people can't be changed, but it's not easy when it's family

‌As I get older, my mindset has become more fixed:
I believe people cannot be changed, and the only thing you can do is gradually distance yourself without making enemies.
I have generally managed my relationships this way to live without stress,
but it's not working with my younger sibling.
If it were just my sibling's problem, I would have cut ties and stopped seeing them at any time,
but because my parents are involved, I feel like I'm going crazy.
A legal dispute arose with my parents, and I have been managing it by following lawyers' advice,
but suddenly my sibling jumped in, creating a battlefield and turning it into a litigation war.
I pleaded through tears, explaining that they shouldn't do this, that it wouldn't provide any relief, that costs would increase, and that the outcome would not become more favorable,
but in the end, even my mother was gaslighted.
She started blaming me and even told me to step away.
I considered doing so, but I told them repeatedly that I could see my parents getting dragged into it and ruined, but there was no way to make them listen.
Eventually, just as I expected, the problem grew, costs increased, and the situation became unfavorable.
The sibling who caused the problem is now throwing a tantrum, claiming they are stressed, and has backed out,
and only then did my mother call me back to ask what we should do.
I am angry, but the most important thing is to reassure my elderly parents,
so I told them I would handle it, and now I am struggling with a problem that has grown too large to manage.
What hurts the most in this whole process is
that my parents, who were once so wise and supportive, have lost their judgment as they age,
and they are showing me weak sides I had never seen before... both physically and mentally.
I know everyone grows old and that's just how it is,
but seeing it happen to my parents while dealing with a sibling's escalating problems is causing me such immense stress that I am even developing depression.