That's really interesting @Intercity225 - we're in the same position as the op having never dealt with a sw before and when we were visited were told a bedroom inspection was mandatory and also that my 17 year olds smaller sibling now has to be assessed as they have to assess all children in the family
Bedroom inspection is not mandatory - which implies its enshrined in a law somewhere; and afaik its not. (Luke Clements says its not; and he is probably the leading figure in this field of law in England). No SW ever asked to see our DD1's bedroom, and she was assessed both as a teenager under 18, and as an adult.
Its true that SS should take a whole family approach, and assess the impact of DC's disability on the siblings (who may be carers by default, when you leave the room) and you, who are very likely to be a carer (you can request a carer's assessment as part of this). If they speak to you, about your caring role, you can insist they do it, not in the presence of DC; and they should send you their questions in advance, so you can think about how you want to answer. However, as per Luke Clements, they can't insist on speaking to siblings on their own - if I were you, I'd insist on being present.
DD2 complained, that no SW ever asked her how she was. When an independent SW did an assessment, one of her criticisms was that SS were totally focussed on DD1's disability and never paid any attention to DD2, who was considerably impacted by it, and was a carer by default.
It is generally assumed by SS that parents will care for their children; but once the children become adults, then no adult in England is obliged to provide any care for another adult. It becomes the responsibility of SS. Obviously, they will try to persuade you to do as much as possible, as its saves them money. You can just answer every question with:
"I am not prepared to do any care for DC..."
You don't have to explain why - its the way to get more help.