What do I do about my sister??? please help :/?

Im sorry this is so long...........My sister is 14 and I'm 16. She's been acting really weird lately. She is freaking out on me and my parents for no reason and is hoarding food in her room like oreos and cookies and has just been so rude. She says she is depressed and shes been cutting herself. I don't think she is depressed though. Most of the time she is happy and making jokes but then something will set her off and its like all hell breaks loose. She claims theres something else that she isn't telling us and won't tell us. She is playing my dad saying she will kill herself if she doesn't get this or that or isn't handed money. She is just using my dad to get things out of him and he's been trying to be so nice to her and then he will take his anger out on me. My dad is taking her to therapy but she just comes home mad from therapy. She can't talk about anything she only yells at us. Today she started freaking out to my dad saying how she needed food in the house. He gave her money for food before work. When i wokeup she was begging to go get food and i said i didn't want to cuz we already have food in the house and I don't want to waste gas because I'm going on empty. She freaked and called my dad and he yelled at me to take her to get something to eat. I told her I'd take her to subway or something like that but no fast food or junk food. I'm trying to lose weight and I don't need or want the temptation. My parents don't want her getting fast food or junk food either anymore cuz she hoards it in her room and attracts bugs and her room reeks. Anyway she said she wanted to go to walgreens to get something to microwave. She comes out of walgreens with a little container of mac and cheese which we already got in the house so i knew she had to have gotten something else and she was hiding it in her purse. I told her that she needed to show it to me and she freaked out and kept saying nothing was in there. Well anyways, later I went in her room and found 2 huge packs of oreos she bought today. I was so mad because she was lying to me all day and she wasn't aloud to have food in her room like that. I took the oreos and hid them and when she found out she freaked out saying how she wants to die and wants to kill herself and they are the only thing that makes her happy and stops her from cutting herself. I told her she lied to me and I wasted my time and gas on buying her junk food which she said she wouldn't and didn't get. I didn't give them back and she stormed into her room. Now all night she's been mad at everyone and yelling and I don't know what to do anymore. Theres no reason for her to be depressed though, we live in a nice house and we are homeschooling for this school year because we moved. She didn't have friends before we moved and she gets everything she could possibly want. She has a bunch of friends she talks to on fb too. The only thing I can think she would be upset over is her weight. She's almost 200 pounds now. My mom has spend 3000 dollars on Jenny Craig for her last year but she gained it all back and then some. Then this year she spent like 200 dollars on another diet for her. She quit the diet and now her therapist says she should eat if it makes her happy. Its all just so confusing.... :( no one knows what to do. Please help.