Is it fair for a teen to have to deal with grandma's brain damage from stroke after having dealt with grandpa's dementia? He is kind.

Not sure. What you mean by "deal with". It is hard to have to deal with the emotional impact of your grandparents' problems. There is help out there. I suffest that you see a therapist to help you work through those emotions. Unfortunately, fairness doesn't enter into it.
Fair? Families generally help each other. No doubt, it's burdensome for you to be asked to help. Perhaps you can negotiate schedules and tasks that suit you. You have a right to your life too - if you're overburdened, speak up but don't expect a complete "pass". It's easy to say "i love you" to your grandparents, but actions speak so much more loudly than words.