If they have not paid some bills for 5 yrs you need to think of either taking professional advice - putting aside your personal feelings - or think about the idea you are never going to be paid. Even in this financial climate houses are selling. If its a case that its not sold and they live rent free or sold and they have to pay their debts - will it ever be sold!
This appears to be a possible verbal agreement, which become very sticky, especially if its a 'within' family matter. Siblings and lynching as nothing to do with the matter. If I was you I would seek legal advice, which might be available from a local Legal Advice Centre or the CAB, alternatively a solicitor who offers the first consultation for free.
Presenting bills (with interest) on verbal promises would have very little standing, unless a written agreement was made. Selling property and trying to obtain money owed or promised could also lead to legal implications, especially if the person doesn't want to sell and possibly become homeless into the bargain. A further point to consider is the length of time the person as lived in the property rent free, which so far appears to be 5 years. Give it another 7 years (I think, but not sure) and they might be able to claim squatters rights.
Apologies if my comments are off track.
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