I found her! Her name is Ginger, by the way. :D
She's under my sister's bed, lying on the spare mattress under there, and her head's on the spare pillow.
It looks like they have a bunk bed. XD Well, my other dumb cat will never find her there. She's too stupid. They were growling at each other from other ends of the bathroom, so we decided to pick up Ginger and put her back in a different room. The dumb cat (Callie) was looking around the bathroom for her. It looked like she was thinking, "Where did she go?..." XD
Also, have you ever heard of Cave Story?