OP, the scenario I face is very common, repeated my times on the elderly parents board. (Do join us granny and others!) I am sure that every unhappy family is unhappy in its own way, and that the roots inevitably extend back to childhood.
(When clearing my mother's flat I found DB's O level certificates which showed he had to retake several. I was off to University, with my excellent exam results, so had no idea. Did I say he resents me?!)
One of the key things about caring is that you may be right, or may be wrong, but you have to get on with it in a way that works for you. From what you have said, you are all probably lucky that your FiL trusts your SiL. You are certainly lucky that she has been willing to take him on.
It is quite possible your SiL is dug in, coping with it all. She probably does not trust you or your motives and may not like you. Fairly, or unfairly "helpful suggestions" don't seem helpful to her at the coal face. It may be too late to gain her trust. There will be things she resents. (Carers allowance is £62.50 pw. Add in £200pw and take away the cost of other bought in care, and I would be resentful.) She may feel that you do not understand what she is up against, or simply want to swan in, swan out and criticise.
A couple of months after my mum had a replacement hip, DB was demanding that I organise gym membership as this is what my mother told him she wanted. I went along with it with predictable results. Only a few weeks before she had got lost in a hospital changing cubicle, and indeed had had to be rescued when she forgot her way home from the supermarket. Even if she got to the gym, there was no way she could use it. Then bless him, he ordered a white board and told me to organise workmen to get it put up on her wall, as it would "help her memory". No way did DM want her living room looking like a conference centre, so I had it put up in the hall. It was never used. Then he says there is no point in him offering to help, as I never listen.
I don't think the Will will be a fight, as I honestly don't care. I know that after my father died DM had wanted to change it, but think she was not sufficiently organised to do so. (The correspondence is incomplete but she was certainly in touch with the lawyer.) I also know that there will be an awful lot more money than there would have been, had I not stepped up to the mark. Indeed had I not done so, I suspect it would be gone long before any Will reading. It took a year of almost full-time to sort out her financial affairs, including a tax investigation, fraud, misselling, carrying out a gas safety check on a rental property and discovering an active leak in a 25 year old boiler and lots more. Three months alone to find and sort the paperwork.
So DB has been lucky. Does he show any appreciation....
A thank-you would go a long way. You might find the same.