Setting boundaries. Help please.

Hello.

I have a Parent who has recently been diagnosed with advanced cancer. They are very scared. They have not got a wide support system, as they do not tolerate people well.

My question is, how do I set boundaries to cope? I have Children with special needs, so life is full on. My Parent has already started needing calls in the night to talk through their anxieties. My child wakes me up through the night as it is, and I don't know how to cope with an extra thing that takes away sleep.

My Parent and I have not had a close relationship, and I have kept them at a bit of a distance. They are notw vulnerable and scared, and needs me as they have pushed away other people throughout the years. 

How do I juggle everything and keep my head above water? 

I've been up since 4 am with my child, and the pressure to message and phone in the night is going to make me sick.

How does everyone get balance in your situations? What can I do do?

Thank you.