B, My thoughts and prayers are w/you and your family. I am so sorry to hear about the health issues that you and your sister are both experiencing.
As for your xh, you've handled his bs extremely well over the years. I honestly do not know how you've managed it w/everything you had to deal w/and continue to deal with.
I'm sending you hugs this evening and warm wishes for everything to settle down for you and your health to improve. Take the relationship slowly and he's not blind...he's more than aware of what is going on in your life and he's there w/his eyes wide open. B, enjoy the friendship and if it's meant to be, it will blossom into something more permanent. Take each day as it comes. Okay?
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.