can't get hold of the person i want to this evening and I may well be on the move for the next day or two.
i'm treading outside my area of not-even-expertise, here, but i don't think there's a need to go through any sort of 'statementing' process to get DWP benefits.
there may be some form of assessment needed to get help from local social services, either in terms of direct help, or financial assistance towards care needs and so on.
the two are different.
there are DWP benefits for people who are, through disability or health condition, not capable of working or seeking work. For an adult of working age, the principal two would be Employment and Support Allowance (the name is fairly typical DWP bullshit - it replaced what was called incapacity benefit, which in turn replaced invalidity benefit) and the other is Personal Independence Payment (this has replaced Disability Living Allowance, which aimed to cover something of the additional costs of either care or mobility needed by disabled people.)
ESA tends to involve filling in a fairly lengthy form (or a carer filling it in for you) - while it can appear quite daunting, the form (ESA 50) is designed to cover almost any possible sort of situation from mental health difficulties to having a limb or two missing and all points between, so it's inevitable that some of the questions won't be relevant to everyone. It also tends to involve some sort of assessment with a 'health professional' employed by some bunch of private sector twunts contracted to the DWP.
There is a long running thread about this
here (ATOS were the main contractors for this) which is a good source of support and info.
Quite how PIP works, I'm not entirely sure.
I'd say that getting partner's daughter on to some form of benefits is a first step.
Caring for someone who is on one of specific benefits is then a qualifier for someone claiming Carers Allowance (think caring has to be 35 hours a week or more).
This then qualifies you for getting a 'carer premium' in any housing benefit calculation - which means the amount of money your household is considered to need before you start being expected to pay anything towards your rent, increases. (if at the moment, you're getting your full rent paid, it's not really an issue, but will be if you start work.)
It can be balls-aching. (or insert suitably gendered / non gendered equivalent if you'd prefer)
It can involve appealing against stupid bloody decisions by the DWP (I think at one point a pretty high proportion of ESA claims were getting refused after the 'medical' then being allowed on appeal - although that only helped people who had the energy and / or support to make an appeal, and people who managed not to die of their health problem before the appeal came through.)
But there is support out there - and not only on here.
Take it one step at a time, and in the words of norman stanley fletcher, don't let the bastards grind you down.
This (national autistic society) page may be a good starting point, if i'm not stating the bleeding obvious. They also have a telephone advice line. They may be better for the specifics here than a generalist service like Citizens Advice.
This is Turn2Us who I mentioned earlier.
Benefits and Work are also good - some of their stuff is open access, some does require a modest subscription.
Again, don't try and digest the whole darn lot in one go...