Please answer! I am begging you!?

I am not my sister Arden who is skinny and everything. I am using her acount. my name is Madelon and i am older then her. i workout 3 days a week swimming for about an hour. i also take 1.8 mile wallks 2 or 3 times a week. i eat about 1200- 1400 calories a day and i can't lose weight. i am on a low fat and carb diet. i don't know what to do. i am older then my sidter Arden who works out 4 days a week and swims 3 hours. she is tall and slim. i am tall and 165 pounds. i know it is disgusting. i have talked to my doctor about this and she said that it is puzzling but i have stunted the path that i was on. 3 years ago i weighed 116. then a year later, 136, then a year later 152, now 165. my doctor says that if by next year if i still haven't lost weight she might refer me to a nutritionist or something. i just wanted to know if this was something common or if it is some type of like disease. i know i am paranoyed but i wanted to know. Thanks!