It sounds as though you are not currently being persued by your elders but that your brother is taking steps to ensure you are Df'd.
As you have bailed your parents-out before, I would be tempted to layback and not be too quick to bail them out again. I know due to YOUR conscience and UNCONDITIONAL LOVE, it's easier said than done especially if you can afford to do so.
As your father said he would pay you back at $100/mo, how does he expect on doing that especially considering he and your mother have limited income? As I assume the debt they already couldn't pay for was for necessities, I think he is not realistic and will likely soon again amass additional dept which he again will expect you to pay. Easy come, easy go!
I suspect that if you were to be df'd, he will feel that he does not need to pay you back as paying a df'd person would affect his conscience and relationship with Jehovah.
As your parents have already demonstrated that their love relationship with you is conditional and as you are already considered to be mentally diseased, I suggest to not provide them any money. You could give the reason that you wouldn't want them to do anything that would affect their relationship with Jah by temping them with money as the fact that you are currently not df'd is only a technicallity as you have unofficially disassociated yourself.
I realy think it's time for your brother to step-up to some family responsibility. He wants to appear so righteous to others but as he is unwilling to assist in the care of his elderly parents, how can he in clear conscience assist the 'flock', many of whom are strangers to him. It could be thrown back at him that as an Elder, his responsibilities begin at home including with his own parents so by not assisting with their needs, he is no longer qualified to be an Elder.