Right, so your brother wants to:
- Betray your DM's trust, act counter to her best interests, and potentially leave her without enough money to cover care home fees.
- Commit criminal offences under the Mental Capacity Act 2005 and tax law.
- Induce you and your other sib to commit criminal offences under the Mental Capacity Act 2005 and tax law.
- Give part of your inheritance (assuming any money is left when your DM dies) to his DD.
I think you and the other attorney need to be on your guard. I'm surprised that you hadn't realised that this would affect your inheritance (not that that's the main issue).
Your DB is prepared to swindle both your mother and his siblings.
Make sure EVERYTHING is in writing, even if you have to summarise a verbal conversation in writing after it happens. Text messages/ WA are fine - in fact, WA is ideal, assuming your brother has notifications switched on, because it creates proof that a message has been read.
The PP warning about an EA valuation makes a good point. Although it's not a legal requirement, it's good practice to have 3 valuations and makes an HMRC challenge less likely. Make sure you and your non-crooked brother instruct the EAs as well and have sight of all correspondence. A good way of doing this is to set up a dedicated email address for your mother's affairs, to which you all have access.
Ensure that any organisation you deal with is aware that there are 3 attorneys, not just your dodgy brother.
Insist on sight of all receipts (you can say it's for tax purposes, which is also true).
Be careful.