Interesting question. I wouldn't say so much their feelings about their parents, but their parents behavior sometimes does have to be a factor. It does get real interesting when both your parents get sick around the same time, you want to help one of them-- do you help the one who lived a more honorable life more?
My mother was a really honorable person who made a few mistakes that hurt others-- but very few, fewer than most. My dad was an honorable person who made a lot more mistakes, hurt more people, but lived very honorably the last 20 years of his life. Did I help Mom more? Just a little bit, maybe, but only when I absolutely had to choose between something she needed and something he needed. I tried to be as even-handed as possible.
When they were both sick, our resources were limited, and it seemed they needed a ton of care. My mom died after running through most of her savings, though there was a bit left over after she died. We stuffed it in a 401K with a good financial planner, and resolved not to touch it-- particularly for Dad's care. Their divorce was very acrimonious; she would have been enraged if we'd spent a dime on him.
I kept flying back east to take care of Dad, and he kept getting in trouble, and wouldn't move closer to us, kept passing mini mental status exams and was deemed responsible for himself, though adult protective services came out several times a week because he wandered. I wondered if we should dig into our own savings to help him, but a wonderful social worker in Maryland sternly told us not to. "Here's how we're trained, and how we tell people to make these decisions," she explained. "Do the best you can, and we'll try to find a group home for your dad. It will cost you some money out of pocket, but don't you dare use your own retirement savings-- if you do, then you'll just become the problem for some other social worker in your state 30 years from now. That would be bad for you, and bad policy."
The time I spent on his care was a much bigger burden than the money, but I took her advice. We found Dad a group home; his last 18 months he was happy and comfortable, in a house without bars on the windows, in a decent neighborhood, with other seniors and staff that was run by someone who actually cared. And looking back on it, I'm so glad I spent that time with him, as hard as it was. He would almost certainly have died sooner, and more unhappy, in a fancy nursing home, doped up on a bunch of drugs.
Just do the best you can, but don't endanger your own well-being or your family's. Hard to know where that line is, but you'll find it. The satisfaction of knowing that you did right by your family is absolutely priceless.