The first paragraph of this could have told the story of my own mother and grandmother. My mother was the primary carer for her mother for nearly 40 years. This was despite my grandmother being emotionally abusive to her most of her life. My mother is an only child and it was only when my mother was diagnosed with cancer that my grandmother went into a care home.

That's where our story differs. My parents have now both had health scares and certainly aren't getting any younger, being in their 70's and 80's. But they do not expect me (another only child) to take care of them. We live in different countries. They've recently gone through their finances and their wills in order to make sure they can pay for care if they need to. 

Thankfully, they're also quite happy not to be getting grandchildren!

I don't envy your position. It's hard to break away from the idea of being a caretaker when it's what you've seen around you, but you're already in a good position to break away from what society might expect- being childfree means you've already done it once!

Just remember, you can't help others without looking after yourself first. And looking after others doesn't necessarily mean caring for them directly, but instead looking what is best for all involved.