No. If she thinks she's ready to live on her own, she's got to provide for herself. There are consequences for every action and, as parents, we need to jump on those opportunities to get this across to our kids.
It's obvious your mom has no control as she allowed it to happen in the first place. Or as you say, she's self-involved, which could be the reason your sister left in the first place?
What about the boyfriend... Is he older? Are they living with his parents or does he have his own place? Does he work and/or provide for her?
In any case, since your sister wanted out, she should be willing to get a job after school and on weekends to fund the lifestyle she chose. Your family may or may not be screwier than another, but your sister still made a choice.
If your mom allows her to come back home, that would be a different story.
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We are always more anxious to be distinguished for a talent which we do not possess, than to be praised for the fifteen which we do possess.
Mark Twain
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