If your mum and dad lack capacity then sadly they are beyond the point of being able to appoint an attorney under a lasting power of attorney.
That being the case then if you need to make day to day decisions or deal with property or bank accounts then the Court of Protection and an application for deputyship is probably your next port of call. It would be well worth check and getting advice on how your parents house is held as between them as part of the deputyship application can involve asking for new legal owner’s to be appointed or making sure the deputy has the power to sell if need be.
It is not as intimidating as it might appear. See..
https://www.gov.uk/become-deputy/overview