state is Pennsylvania and I understand about getting a lawyer...just found the papers and figured someone would at least have an idea if it was even able to be pursued.
Most contracts have a “Governing Law” clause, seek a lawyer in that state. Most states have a “Statute of Limitations” which partly took over from the common law principle of ‘Laches”. Talk to a lawyer and try to figure out the economics of pursuing an action.
I can only go by your description of the situation, which is a potentially fatal defect in my take.
If a schedule of payments was created by arbitration, (which means “binding” arbitration), then stopping those payments would generally and generically be be regarded as a default. But the payees (your folks) may have forfeited their right to collect based upon their failure to assert said default in a timely fashion. The contract should have defined what might constitute a default and what steps your folks ought to have taken; but nobody can say which way those things are without reading the contract.
I searched for “statute of limitations debt pennsylvania” and it seems like a SOL of 4 or 6 years exists. Of course, 90+% of all such listings will be written from the standpoint of the debtor. But there appears to be law written on your precise point in PA.
https://jbmartinlaw.com/dont-wait-too-long-to-collect-a-debt-in-pa/
Ergo: “SOL” could mean two quite different things in your case.
Maybe. Go see a lawyer. They’ll give you the first half hour. They’ll look at contract and tell you if it’s worth the tome