This reminds me of Jesus rebuking those who were saying they couldn't help their aged parents because their money was dedicated to the temple (corban I think.) If it was wrong then it is certainly wrong now. Your brother needs to consider his financial obligation to his parents before reaching out to the circuit work. If they had given to him out of their surplus then he may be able to forget it but this came out of their need. The man has no conscience.
I expect you will fume and give them the money in the end. It says a lot that their JW son is more selfish than their non JW one. What I would do is ask your parents if they would sell you your brother's debt, then tell your brother he is in debt to you and as you are an evitl apostate he would not wish to be embarrassed by such a financial obligation and would need to pay this off as soon as possible.