They get that in the future. Its not discoverd yet. Priscilla knows about it because she went back in time.
Step-mom spends too much, so much that whatever assets they have can't cover their debt. That, and stepmom must not be too bright. MC did mention how she doesn't care about anything but the jewelries and other material items she could buy.
excessive spending and failed investments. after all, it takes a lot of money to take care of a mansion and the trash the stepmother buys can't be cheap either.
Why would they be in debt . . . . if they have a diamond mine . . . . . ?