Loaned parents money, how can I ask for it back?

Summer '02, found out from ex sis-in-law that dad's truck was getting repo'd.Called them and offered to help them out.They agreed, I put $3500 on a credit card to pay off the truck and prevent repo.Stepmom promised to pay the monthly credit card bill. First bill comes in, called to tell them, nothing. I paid that month. And I did for next 5 months. Christmas time, I get a check for $500 for missed payments. That was the only time I got money from them. I have since paid it off. Dad's 64 still working & heart troubles, stepmom is currently on disablility. she was working when I bailed them out. Summer '07 she started getting $850/mo disability. Blew first ck my two nephews. Nephews had great christmas with $$$ gifts/toys from them. I'm pissed off... she is spending money like crazy and i'm not getting a dime. They rent & have nothing, filed Chapt 7 several times. My guilt.. asking them for the money and sending dad to a heart attack or to his grave. Am I too nice or a doormat?