My sister is Anorexic but my parents won't believe me?! HELP?!?

OK, this is going to be long... So my sister was acting really odd for the past few months right after our trip to Disney World. She reduced her food amount drastically and even cut out the food she can't live without: Potato Chips. She started going to her room as early as six o' clock and go to sleep as early as seven. So, that means she hasn't been eating dinner. Before, she would look in the mirror every morning and suck her belly in. She did have sort of a belly before, but she wasn't overweight. Now, she has lost a lot of weight. She skips breakfast (nothing new), but then she will remove stuff from her lunch kit which my mum packed, even when my mum put less so my sister would finish it. She didn't. Then she would come home from school and say she's not hungry and go upstairs to read or, until recently, play on her NEW iPod Touch. She started drinking lots of water (she never did before) and diet fizzy drinks. She complains of being cold at warm days. She once took a potato chip and put it in tissue paper and said "Eww! Look how GREASY it is!!". Whenever we make things at our cooking classes, she'll tell the kids she can't wait to eat them, then she'll not touch them at home. I am writing this right after one of her tantrums. She went to her room right after coming from her friends house. I went upstairs and asked if she was going to eat dinner with us. She said she already ate at her friends house. I asked what. She said two slices of pizza. So I said I would ask my brother who was also there, then she admitted she didn't eat anything. She promised she would eat when my mum came home. She didn't eat. I made her some lasagna (she used to LOVE it) and she promised she would go to her room and eat it when it was finished cooking. She went to sleep. My mum made her come downstairs (she didn't want to waste the food) and my sister was screaming and crying and ended up not eating. By the way, I had Anorexia before, so I know the symptoms. The thing that bothers me to the point of complete utter belief is that my parents don't believe me! They say this is normal behavior, or she's growing up, or she's trying to annoy me to get attention, Blah blah blah. I need advice on how to convince my parents she has a problem and to get her into therapy