Helping a gambler to repay debts is never helps them, it only give them a clean slate to gamble again (just as Velvet has stated in her reply). I know from my expereince when I helped my father with debts, he would immediately forget the ***** I helped him and do it all over again.
Gamblers cannot be trusted with money and that is why we cannot help them. They need to pay their own debts, regardless of who is after them. They knew from the start of the terms of the loan and yet they chose to gamble it all away. Do not feel bad about not helping either, because they should be the ones feeling bad about asking you to give them money for something so irresponsible.
I know this is hard and frustrating because you are dealing with family, but you have to remember you cannot give life to something that is so destructive as gambling.