Your aunt is fortunate to have someone like yourself who is concerned for her welfare. I don’t know the law in the UK so can’t advise you about your rights as a concerned relative. I don’t know if and how you can “force him out of the house”. A lawyer who specializes in elder care would be most helpful.
You didn’t share your aunt’s age. If she is a competent adult, she is entitled to make her own decisions — even bad ones. However, if she is elderly, what you are describing may meet the criteria for neglect of an elder. To find out what you may be able to do about it, see this link on the Gov.UK website: https://www.gov.uk/report-abuse-of-older-person.
By all means, inform her adult children of your concerns. A family meeting to discuss how best to help their mother as she declines would probably be a good idea.
I wish you well.