Having experienced this myself, I'd tell you to run run RUN away from taking on that additional responsibility. I was left to care for a difficult, mean and sometimes loving (confusing, huh?) mother who essentially was capable of doing a lot for herself, but found it easier to get me to do it. I think the ultimate aim was to get me to move in with her, leaving my own husband and family home to focus on her. Two sisters, one overseas and one an hour away, both full of "helpful" advice and suggestions for how I could do things better. Both sat on their fat arses whilst I quite literally shredded myself trying to run my life and home, my own business, tackle serious health problems of my own, and live entirely at our mother's beck and call for a decade. My relationships with them will probably never recover and to be honest, I don't want them to. I despise them both. I received regular lectures on what I was doing wrong, dealt with vile accusations (told them to go to the police, funnily enough they didn't) and spent ten years being told that I wasn't doing enough. I could have bled to death in front of them, they didn't care, as long as they weren't doing the work themselves.
There is no obligation to care for our elderly relatives. We impose it on ourselves, or allow family members to "choose" it for us. I wish I'd been stronger, but it crept in....slowly but surely, we went from taking her food shopping and giving her lifts to appointments, to answering "emergency" phone calls from her at 2am, spending nights in A & E for "heart problems" that turned out to be indigestion, leaving our own meals because she was hungry and wanted a sandwich made, NOW. If we didn't drop everything, she phoned my sisters to complain that we were starving her. We involved social services, mental health professionals, her GP practice.....it was all determined to be manipulation. I wanted to smother her with a pillow for years. I wished for her death. I really did. I hated her, in the end. I had a nervous breakdown when she finally died.
Do. Not. Do. It.