Use this as your vent. It can be therapeutic to write your feelings down and it sounds like you need that right now.
You can write as well so give yourself that!
Take care of yourself while they are going through this process. It will look up and it will get better.
I know it seems impossible and scary now but give it time. They need time to find what makes them happy and whole apart from each other but once they do you and your sisters lives should dramatically improve.
I have just spent 12 min trying not to say something horribly depressing as a reply.
I gave up.
I wish I had something to reply that is not depressing but I cant so all I can say is thank you for the kind words.
If that’s what you have then say it. Depression ebbs and flows. What you feel now will pass... It’s just hard waiting for it to pass.
Life and stuff.
Sometimes it does