I'm fairly certain that my DW only has one sister, but hell, your DW could easily be my wife's long lost sister.
Sounds exactly like what my wife has done in the past and always uses the Scrooge and cheapskate line on me saying that she works also and blah blah blah.
It's soooo aggravating trying to explain why she shouldn't buy this and that and that we have other bills to pay besides her Amazon addiction (yes, it is an addiction no matter how much they refuse to accept it).
I feel for you, I really do. Here's to hoping that your wife comes around one day and sees the light.