But how do you go through life not trusting any adult? I find it more bizarre to assume all will not be OK and that all males are potential rapists/abusers unless they can prove otherwise (and how exactly are they supposed to do that?)
How do you teach your child when it's OK to trust a stranger (eg when they're lost or in an emergency) if your default position is "Don't trust any adult ever"? They're the children who are the most vulnerable as they haven't been taught ways to determine the difference between a "good" adult and a "bad" one.
I was raped at Uni, so I know that there are men out there with bad intentions, but that doesn't mean that I teach my DD to treat every man as a potential rapist. I was raped in a park in the early evening, I haven't taught my DD that there are potential rapists hiding in every park, but I have taught her how to keep herself safest if she finds herself in the park alone in the early evening.
My DD has been taught what behaviour is OK from an adult to a child and what is not; she knows what to do if she ever feels uncomfortable/unsafe in an unfamiliar situation and I've taught her how to assess the most safe people to approach if there's an emergency but I haven't taught her to avoid any male over the age of 16 "just in case".
By all means if the family in the OP was one where the older son was known to be a bit of a tearaway/hanging with a bad crowd/into drink or drugs/been in trouble with the police etc where the OP might have legitimate concerns then I would understand the hesitation but the fact that an 18yo older sibling who happens to be male is in the house should not be a blanket no.
We wonder why women are often left to do the childcare but then any male who is even in the vicinity of a child is treated with the utmost of suspicion even if they're just living their lives in the same house ... This poor 18yo is just living at home with his mum and sister and yet is a potential abuser in the eyes of many MNetters - if he was an 18yo sister nobody would bat an eyelid, but because he's a male then he's treated with suspicion when he will probably want as little to do with his 10yo sisters sleepover as possible.