7 year old refuses to flush the toilet.

My 7 year old sister refuses to flush the toilet after she uses it . It’s literally disgusting and stinks up the whole house when she does it. I’ve told her millions of times to flush after she uses the bathroom but she doesn’t listen instead she just continues to do it. I’ve told my parents and they know about it too but don’t do anything but tell her “just flush the toilet” but she still doesn’t flush. It’s literally so gross and the whole house stinks like shit every day because she constantly does this. My parents don’t trip because by the time they get home from work it’s already been flushed and candles have been lit and windows have been open to flush out the smell. Do other little kids do this ? How do you get them to stop ? shes 7 about to turn 8 .