My father died when I was young, and my mother while pushing me academically and giving me many opportunities in the arts and academia through tutors and books, was a cold and distant parent.
I left home at 17 and through various points of my life, because of her chronic ill health I took years out of my youth and adult hood to care for her, which cost me dearly in my career, personal life and finances, with zero thanks from her.
Your DH did not have a close relationship with his parents, his mother may have ‘had to focus on her husband’, but not to the emotional developmental detriment of her son.
His sister chooses to be carer, your DH for reasons of self preservation does not. Leave you DH to it, or if you must niggle at it, get a couples therapist, so they can unravel for you, why him being involved is a bad idea.
I wish I hadn’t cared for my mum all those years, I regret it. I have no regrets for the years of care I gave my father as a child, as he was loving and demonstrative, but my mothers love was conditional I don’t think she ever should have been a parent.
I’d leave it alone. If sister can’t handle it, then they need to go into sheltered housing.