It's fairly common, on this site, for people to ask questions in this form: that is, to describe something that someone in authority - a judge, a police officer, a parent - has done, and to ask, "Can they do that?" This is not a useful way to frame the question. You already know the answer - manifestly, they "can" do that (whatever it was); if they actually did it. The real question is, was the action legally proper, and, if not, what can be done in response?
Parents cannot legally neglect their children. A person is a 'child' until they turn 18 - they are limited in their ability to get jobs, find housing, and so on, and if a parent knowingly refuses to provide them with adequate care, then they may face criminal charges, or get a visit from Child Protective Services (CPS), a division of the Department of Human Services (DHS). DHS's response will vary a great deal based on the particulars of the case. They may ask the District Attorney to file a dependency petition in the juvenile court, trying to take the child out of the parents' home and place them somewhere safer; they may also provide some other 'services' to the family, to try to get them to be able to go on independently. DHS is so short on resources that this latter solution has a lot of appeal to them.
If the child does need to be placed elsewhere under state authority, then placement with other family members is preferred, if any of them are available and suitable. This brings me back to a question I've been wondering about - if this is your step-daughter, can you (or your spouse, who, I presume, is the child's other parent) take her in yourself? One parent's neglect can be the basis for a change of custody, if the child's father wants it. As a step-parent, your own ability to act is very limited. You have no particular custody rights over the child, unless you can show that you've been raising her as a parent would for an extended time - which doesn't seem likely, as you describe it.
I would advise the child's father to consult with an attorney in private, and advise the child to talk to a school counselor or other official. I would be glad to discuss the matter with either of them, but I couldn't do both. If the father and child don't want the same thing, they can't get legal advice from the same lawyer.