She will have to be properly served with the legal papers, probably by the grandmother.
If proper service is on file and Mom isn't there, I would give GM good odds on getting custody, if that is what you and your cousin want for your own children.
However, it she was not propertly served with the legal papers because GM does not know her address, etc., it is possible the hearing will be continued until she is properly served.
If she flees with the children, you, your cousin ,and grandmother could well be dealing with law enforcement regarding their return to the jurisdiction of the court.
But what are the chances that she can get buy with no child support, etc., for the two younger children and a BF who is a small time drug dealer who otherwise works minimum wage jobs?
Moreover, it is really very hard to disappear completely in today's world and not be found. It just may be expensive.
If GM gets custody, at some point, you and your cousin likely will be ordered to pay child support to GM. You also will want to establish a visitation schedule, even if you otherwise trust GM to allow you to see your child. If the older child is removed from her custody, the chances are that the CS being paid to her will be paid to GM instead, especially if being collected via a wage garnishment.