NIECE CAN'T FLUSH!

Jasmine
My 8 year old niece has the habit of not flushing or washing her hands... I made 2 giant signs for the bathroom in the common area and in my bathroom... I asked if she's afraid of the sound the toilet makes, she said no I just forget..... I guess her parents don't teach her about personal hygiene. I constantly remind her. Idk how to break this gross habit?!