Hi,
A hip x-ray should confirm whether the problem is Pethe's. If the blood supply to the head of the thigh bone has been compromised, the ball of the thigh bone may start to alter in shape,m and not fit the joint correctly. With the family history, it is important to have the corrct treatment. I suggest you obtain an x-ray, and if confirmed, seek advice from a paediatric expert. Other hip joint diseases can cause problems in children, so a diagnosis is needed before treatment is instituted.
Perthe's causes are unknown, but can be more common in families, but this does not mean that someone with a family history will necessarily develop the problem. What causes the disease to be expressed or not is unknown.
Hope the above helps. The sooner the disease is treated, the better the results.
Regards,
MrPhysio+