My stepsister angry with our whole family. She says we didn't look after her. She is not listening?

try to comfort her and make her feel like she is worth every second of your life
make her feel like she isn't just a STEPsister, but a real sister. treat her the way you would want to be treated in her situation after understand that specific situation.
take her to places she enjoys and loves
if you cant convince her, keep in contact with her friends so that they can lure her into a place she enjoys with you and the whole family
surprise her
ask her what she means by 'not looking after her' so that you can fix it
or
give her some space for a while as she doesn't want to talk to you and then try to be by her side
im not you to know what it's like but i hope this is helpful