I'm sorry for a late update - I'm so thankful for the supportive posts, I didn't think I'd get many responses and assumed they'd be in mum's favour so it's been a lot to process. I've taken it all board as I do have a personality that's a bit soft.
Firstly, I did manage to speak to my brother briefly, he phoned back before I met with mum.
I asked if he'd chatted to mum at all recently about the inheritance gift, he said no, and asked me why I was asking; I was honest and told him.
He sounded genuinely surprised, and said that it's not fair of her to ask, he made it clear he wouldn't be gifting anything back at all. I asked him about potentially both of us gifting X amount, but again he said no - he was very adamant, and said I should say no too.
It did feel a relief to speak with him, almost having permission to say no.
I met mum l, she seemed fine, cheery, and so I asked a few questions about what she has been up to, and if she have anyone new in her life, etc. She does.
I gently delved further and she said a few things that have made me very, very concerned. A man she met at her hobby (the shared hobby with my brother) seems to have almost wedged himself into her life overnight.
We didn't talk about the gifted amount at all - I was so shocked hearing about his new man and gently probing, I think I'm safe in assuming he has everything to do with this sudden backtrack of the inheritance money.
Afterwards, I phoned my brother again and he's furious/concerned - so we're both on the case and looking to the next steps to protect mum.
Sorry this is lengthly - and thank you again for the support, you may well have saved me just going along with this and losing my home. And potentially my mother losing more money too (some is gone on this man).
I know con men exist but I can't quite believe if mum has been taken in. She's not a vulnerable woman - very independent and intelligent.