Well see, my sister is in foster care and she is 17. From my experience being in Foster Care once you turn 18 you have legal rights to go out on your own. I am not sure what a D.A.L is but my sister just recently had a meeting with her D.A.L letting her know that she is not leaving when she turns 18. I try talking to her case worker but she never answers. Any help would be great!

GAL? Guardian ad litem.
When she is 18 she is free to leave.
It's not clear from the way you wrote who said that isn't true - or did she just say she is staying on by agreement with the foster parents? They will just lose their funding when she's 18, but no one can tell anyone where she goes, or if she stays.

It's possible that her state has a policy of continuing foster care if senior year of high school finishes just after turning 18.