Hi, I'd like to help you with your question.
It sounds like your ex is playing both sides of this situation by staying with this other person but contacting you for venting and being cared for. Your ex needs to decide which person he wants to be with, good or bad.
Often, a spouse that leaves a marriage may try to stay connected to the partner they are divorcing. They do not want to leave the comfort of the old relationship and take the risk a new relationship demands. So they play both sides, using the people involved against each other. He will tell you all the bad things he experiences with this other person as a way to vent. But if it was all bad, he would leave. He may also be doing the same about you with this other person. There is no sure way to tell.
What you can do is confront him. Let him know that if he wants his relationship back with you, he needs to end the other one and come home. If he doesn't want to do that, then you need to decide if you are comfortable with him using you as a sounding board and a safety net in case his current relationship does not work out. Either way, a decision needs to be made before the situation gets worse. You have a lot of stress with caring for your sick daughter and caring for your granddaughter. Settling this problem with your ex would help alleviate some of your stress.
I hope this has helped you,