This is general information and not legal advice. No attorney-client relationship is formed. This is for educational purposes only.
Your niece may be able to get a deferral or continuance, but only if she appears at that hearing. However, if she earns under a certain income, she may be eligible for court appointed counsel in her jurisdiction. Your court's system may not allow for appointed counsel until your niece shows up at the court hearing.
It is important for her to go to the hearing for a couple reasons: 1) it shows she cares and is responsible (even though she may be scared and worried); 2) this is the only way for her to learn what goes on at this hearing; and 3) she has the opportunity to have a say in what happens.
That being said, the initial hearing is likely to focus on two things: 1) temporary placement of the children, either with one or both parents, a relative or in a non-relative foster home; and 2) jurisdiction. Determining jurisdiction means a court's finding if either or both parents neglected or abused the child/ren such that the court exercises authority over the children and parents. Also, it is possible child welfare services (CWS) will offer your niece a service plan, although sometimes that step doesn't occur until a later hearing, or is offered administratively by her CWS worker.
You are correct that if your niece is not a minor, she should've been served personally. However, the way the court will look at this service error is that your niece has actual knowledge of the court date, so has no reason to miss it.
If you niece does miss the court date, she will be defaulted and the court will proceed without her.
Although it is very upsetting to have CWS and court involved in her life, your niece will do best if she remains outwardly calm, polite to CWS, the judge and all other court personnel and willing to begin services.
If you would like to provide more information, if you have other questions or if I can be of any further assistance, please just reply to continue.
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Best wishes to you and your niece.