I think most people with a healthy relationship will do some things for their parents as they get older. But there is a line where it becomes unreasonable - imagine that your parent’s live in a place you don’t like being in or get benefit from going to (which may be how your brother feels) and you are expected to give up all of your free time to do those things. I’d be pretty resentful of being expected to give up all of my own free time and happiness to be a worker bee for my parents. It sounds like you are currently in a position where you consider the balance between what you are giving and receiving to be fair, which is a positive thing for both you and your parents. But where you have mismatched expectations and priorities (which could be the situation with your brother) it can really tear relationships apart. (I’m not saying this is the actual situation in your family, just projecting a bit and using it as an example!)
For my situation I think caring for my mother will work out well for both of us and should actually be financially beneficial as well. My sister on the other hand is in a completely different situation - she and her husband have been working towards being able to partially (or even fully) retire early and are looking forward to being able to enjoy their free time. Expecting her to give up what she and her husband have been working towards would be way too much, whereas for me giving up a job I’m not enjoying that much these days is a bonus! I’m anticipating my sister being the “fun daughter” that pops in to take mum out to lunch occasionally while I’m the one doing all the hard work and cleaning 🤣 . But that’s a role I am choosing of my own free will, not one that is being forced onto me. If it did not suit me we would work out other solutions.