I'm not in quite the same position as I live close to my mother (86) and help her every day. However, I have the same comparison issues as I'm not able to do as much as my sister. As a very late addition to the family I still have dc at home, one with additional needs, a part-time job and a dh who works constantly, so all housework, cooking, etc. is up to me. My dsis's children are adult (the same age as me) and she's retired. She does, however, have a serious health condition, but battles on and cares for Mum.
Mum herself understands that I don't have the same amount of time, but because I do have better health than dsis she constantly worries about the fact that dsis does it 'all on her own'. This isn't true - I go to Mum every day. But Dsis does a lot more. She rings me every day and tells me all the things that she has done, does things like ringing when I'm preparing a family meal (dd has food/weight issues) and getting annoyed when I ask if we can talk later. Usually adds that she didn't have time to cook, or even eat, today. I don't have that as an option.
I feel I should be there more, and doing more. But Mum was ill recently and whilst I was 'distracted' by that, my dds weight dropped down into the red zone as (although she's 16) she won't eat without constant nagging. I feel like a failure both as a mother and a daughter at the same time.
I try to tell myself - and would suggest to you as well - that the point is not comparing what one person does to what another person does. It's whether the elderly person is getting what they need, and that this is not to the complete detriment of any family member. Whether or not I can take my own advice is a different matter!