Yup. Time to let go. YOU meet your own wants, needs, and limits and put your own oxygen mask on first.
End some of those bonds. Stop being lovers, stop being marrieds. If you still want to participate in this family like a family friend /uncle person? Keep conduct in the sister/friend bucket so you can detach from each other romantically more cleanly. You need to be free to find new romance. Simplify life for everyone. Don't be sleeping with your sister. (metaphorically.)
Could give her the gift of a LIMIT reached. YOU end it because she doesn't have the strength to.
She will feel sad. So what? She's already
sad. Could stop getting distracted with her WANT to be together (which can no longer be because limit is reached) and listen to her NEED to be free of suffering.
Suffering will end when she's completed
the journey though the stages of grief.
Don't come back to wear roles that you know do not fit you any more
just because she asks you to when she's in the denial/bargaining stage and you feel uncomfortable watching her process through the stages of grief. You are not comfortable NOW. So pick the path that leads to your improved wellness in the long run then.
Giving false hope just to come back HERE again and have to take another running jump through the stages of grief.
Ugh. Prolongs her suffering through the Grieving Time because she never gets to FINISH. And why prolong suffering for you? That's not taking care of you.
If you do not know what to say in a stage she happens to be in, say nothing but "I am sorry. I grieve too. I see you grieve. Limit has been reached. It is what it is and it is nobody's fault that grief process just feels... unfun."
Give it the time, and you will ALL make it through. Keep going back to square one? You walk in circles.
Sigh. Hang in there.