J.F.
You are right, most babies' heads are steady by the age of 5-6 months. Your sister's baby should have had several well-child checks (which are usually free) by the time she is 1. Your sister needs to schedule an appointment with a pediatrician immediately, and voice her concerns about her daughter's head issues. If she isn't getting any answers or solutions, she needs to take her to a new doctor.
Best of luck to you all!