Money burns bridges
Ask your uncle the amount he wants.
You stated this on your question, so you should've known by now..Pay back your uncle whatever the amount your mother owed him, according to the contract.
After that1, calculate the difference between the amount he wants and the amount already repaid.
Talk with your brothers how much amount (all of) you can and willing to spare for your uncle.
Explain to your uncle that the contract has been honored, and this is an appreciation for what he has done.
Don't mention any of his mortgage. If he brings it up, thank him for going so far to help your mother, and politely remind him that his mortgage is not in contract and is his responsibility. Avoid further arguments.Give2 the money to him.
1You must finish the contract first before going to the next step. This is to ensure that your responsibility is done, and your uncle can't bring it up in argument.
One scenario is your uncle might refuse the payment. Simply transfer the amount to his bank account, and it is a proof that you have paid him. Whatever he will do to that money afterward does not disprove that you have makemade a payment, including transferring back the money.
2Not pay. You don't owe him anything after you honor your contract.
You are not responsible to your uncle bad choices. This is basically a step-by-step to solve this and avoid complication on your morality. Stop feeling guilty.
After this drama, you can offer help to your uncle to get through his debt. Personally, I prefer gifts (as money), as you will be prepared not to see your money back, but you can also offer loan, with contract.
Remember that "money burns bridges". Everything will never be the same. Whatever your other relatives might think of you, don't feel guilty. You've completed the contract, and you already offering help to your uncle.