POA or equivalent sounds critical.
Has your dad made out a POA in favour of your step mum? If not you have a problem and may need to apply to the Court of Protection as he is too late to grant it. If he has, you may be in just about enough time for your Step mum to grant POA to you.
You dont need to use a solicitor for a POA but given you are dealing with someone who already does not have capacity you may need to. If he has given your step mum POA then you could look on the OPG website and perhaps just fill out the forms so her granting of POA to you is done. You could ask in the legal section of the Alzheimer's Society how you take up POA for you dad given it is passed through your step mum.
Sheltered sounds like a good idea. (Very sheltered - do not think about "retirement homes" of the sort marketed by McCarthy and Stone. You want somewhere with a 24 hour warden and ideally a restaurant or a coffee lounge.) Get your step mum hand over paperwork to one of you. Honestly my mum made such a mess in the two years when she was first widowed and trying to cope without a memory that it took me over two years to unravel and we were very lucky that "fraud" was in the thousands not tens of thousands. Talk to your siblings about how you might want to share the burden. One doing finance which can be done remotely, another organising care, another taking on more of the visiting or helping with the property transactions. Dont expect it to be even. It never is.
Then do your sums:
- can she afford to move without selling. (We essentially gave my mum a bridging loan by remortgaging as the money was there but inaccessible in the short term.)
- what properties are available. Good ones often have waiting lists. Get three estate agents in early to look at how much it is worth, and how much work/decluttering is needed to maximise its value.
- what are care costs likely to be. Is there likely to be any help from Social Services. (And in the short term, until any POA/Court of Protection stuff is done, will they need a SS loan against the future value of the house. Really worth having an early dicussion with SS once you have worked out what the best approach legally is, not least because they should know about sheltered options.)
In short the first step has to be to find out how much there is available and take steps to ensure it is accessible. Then you can look at options.
The will is a different issue. In your case it probably needs to be done as I assume that if your father dies first, everything passes to your step mother and perhaps then to her children, bypassing you.
Unless they are well over thresholds, I assume that your father making a will is less important as everything just goes to her.