Unfortunately intervening will probably not get you the results that you say you are looking for, therefore I respectfully suggest that you stop yourself from doing so.
Your sister will probably not benefit from financial advice because finances are not really her problem. Your sister is searching for something. People who live that way are sometimes looking to fill a void in their lives. They search for happiness in a job and it is not there…they search for happiness in posessions and it is not there…they search for happiness in all kinds of places but because that kind of fulfillment is only found in Christ Jesus then they are always coming up empty.
Your sister needs love and lots of it. Try and be a source of this love as best as you can. When your sister is talking about making another purchase…instead of getting annoyed with her, try and see past all that and embrace her with love because that is really what she is searching for.
Your parents are doing what many parents do as well. Pray for them too.
The joy you have found in Christ is a blessing. Your sister has not been given as much of God’s beautiful grace as you have been given…what she really needs is your sympathy. Not earthly sympathy but spiritual sympathy. Try and see her in this light instead of seeing her as selfish and irritating. It will be hard to do at first but I urge you to try. This helped me in ways I could have never imagined. Hope this helps.