Ghost? Demon? What Is Going On?

Yesterday, my sister woke up and witnessed what she believed to be an evil spirit. According to her, it took the appearance of an "onryo", or vengeful spirit, possessing a white gown and red eyes. It was standing nearly as high as the ceiling and was slouched over. It was estimated to be 7-8 feet tall. My sister claimed to have a feeling of dread upon seeing the figure. Note that I was asleep at this time, and my sister had been sleeping on the floor. At first, I didn't believe her at all. I thought she was seeing things, but then I began seeing things. I still dismissed it as natural.

However, ever since today, my sister gets these crippling stomach cramps whenever she enters my room (where the sighting took place). The cramps go away whenever she leaves the room. I do not experience these cramps, and my sister never pulls pranks or anything of the sort. I recently caught more glimpses of the figure, but nothing much else has happened to me, aside from feeling dread as well. Our other friends didn't believe us when we told them. After a long time researching, my sister's problem still persists, and is starting to get worse.

My family is starting to have family problems and arguments as well, and I feel that it will continue to get worse if action is not taken. Is it a ghost or a demon? Or could it perhaps be naturally explained? Because at this point, I am just not sure. I just hope we can remove the problem and continue on, because everything is starting to fall to pieces. I cannot guarantee that my experience is paranormal, but I can definitely guarantee it is causing a lot of problems and issues throughout. Please help, thank you.