Lpa suitability of attorneys

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Hi all, my husband and I are setting up health and financial lpas for his mum who is 88 and still 90% all there. She asked us to after the awful problems her brother had over his wife's care. The problem is that my husbamd has a younger brother with severe mental impairment due to a combination of ptsd and schizophrenia. He has a council tax exemption so it's officially recorded. Can he be an attorney for her if he demands to be? We would rather he is not as he can be quite unstable and irrational at times (he is never ever violent and is only a danger to himself). Ethically we have to inform him as he is her other son. Could he challenge it and would his mental impairment disqualify him anyway? There is nothing out there even on the govt sites.
No, as he "lacks mental capacity" I'm sure he cannot be an attorney, although he may still have to be notified as he is close family. Your husband can act alone, you could also be an attorney, or you might like to appoint a solicitor if it is a large estate and there is the possibility of some squabbling. Under these circumstances I would definitely getting a solicitor to draw up the LPA, so he/she can verify that mum wasn't coerced into having you as attorney.