If she had plans for this, how did she think she would dispose of their bodies?
She wouldn't have to dispose of her parent's bodies at all, if she made it look like a burglary/ home invasion.
She could chloroform them while they slept, break their necks, or anything while they were knocked out.
She would just act like she came home to discover them dead, after it happened. Then she would call the police and try to act devastated.
After a very short 'grieving period', where she is showered in attention from sympathetic friends, she could turn the house into party central. She could move Amy in as planned, and have a different guy every night of the week, if she wanted. She would feel so 'grown-up, with her own house, multiple cars, maybe even life insurance pay-outs.
I think this was her fantasy, anyway. Never mind the house wasn't paid off, and without a job, she would have no way to make the mortgage payments. She doesn't understand 'real life' money basics.
Anyway, that's pretty much what I think she wanted to do.