How much do you want to bet that your dad will STILL want the money after you are DFd. Sure, his conscience may bother him, but the bills...He'll want the money still. And will want to shun you at the same time.
I certainly wouldn't give them the money. Let the others in the family who are not being shunned deal with it. You'll just be lightening the load of those who are treating you like crap. Your brother may have "obligations" but what choice will he have? He knows it's in his best interest for you to fork over the money ASAP.
You don't owe them a thing, so don't give them anything. Let your brother have to earn some money to help them.