Should I back away?

That's a toughie - I've been where you're standing.

My advice, if you and his sister can communicate, lay it out for her..."I'll help, but I am not signing on for a half-share..." if that's what you want to do - or, if that's not what you want to do, "If you get stuck and need an afternoon off every couple of weeks, If I can, I'll come for a few hours."

Oh no! I'm so sorry, that's a horrible situation. I don't believe you can find the right answers here from a bunch of strangers. I think you need to listen to your gut instincts. One hand you could end up a care taker, the other you could feel guilt & regret. The poor guy is suffering and has no one to help, its heartbreaking. If you can find a way to be there for him without being roped in to full time caring, you just need to stay aware at all times. If he is your friend he should understand that but leaving him on his own to struggle is honestly heartbreaking. If you think you can have an open honest convo with his daughters then that might help?
I wish you and your ex all the best 💞

Only do what your heart and mind want to do. You do not have to help him anymore or less that you would help someone else. His situation is unfortunate but one of you decided to leave the other.
If your love draws you to him fine. If it doesn’t, fine. You can love yourself and someone else—or not. It is on him if he has daughters that are useless.

Help him..in the end, it's all that counts.

I think if the two of you are still friends and he likes you keeping in touch with him then you should stay in his life. He might feel abandoned if you didn't continue now? Maybe his sister can take the lead in caretaking and you can help leaving you not the main person to be the caretaker?

I was thinking to help him until I saw you said you didn't want to because you wanted to live your own life. Don't do it then. Your life comes before anyone elses, you only get one

Help when you can but not financially and know where the line is between care taker and friend.

You can help the sister but tread carefully not to get drawn in too far.

What is the harm in doing so?

Since he has family you should feel no obligation