My sister in law has ruined their finances.

Don't give them any more money. Full stop.

I have a brother like this. He would lie and use his kids to guilt me into giving him money. It will never end unless you end it. You gotta just cut them off and let them fend for themselves.

Your brother his a grown man and should be able to fend for himself. He needs to get control of the finances away from his spouse and either work out a budget with her and oversee the spending, or cut her out of budgeting responsibilities all together if she is unable or unwilling to cooperate and control her spending. Give your brother this advice and be done with it. You're not responsible for their situation and you shouldn't be made to feel like a bad relative or feel guilty for refusing to flush your hard-earned money down their toilet of debt.