Can a family member be held legally responsible for their part in abusing credit cards of a 91 year old with Dementia?

My sister had Power of Attorney over my 91 year old Mother''s estate. My Mother has sight problems and dementia. My sister recently passed away and we found over $60,000 of credit card debt, liens against my Mother''s house, and unfiled tax returns for at least 3 years. My brother-in-law did not have legal power of attorney, but contributed to the debt considering he had no job and he used the credit cards. But the 2 of them hadn''t paid rent for years, had no jobs, and lived off my Mother''s income. My brother-in-law still lives next door in my Mohers duplex. Is there any legal action we can take against him for this financial disaster?