I'm glad that 'ranting' has made you feel a little bit better, I took my time to read your post, even though right now you might feel like 'what's the point anymore, what am I doing with myself?', things WILL get better - just in time.
I've had similar issues with weight - although I've improved and I'm a healthy weight currently - I'm a 46 kg but I'm 16, and I'm really small (height), which is healthy. My tummy isn't flat either - I swear it's impossible to get it perfect - and I compare myself to my friends, who are sporty and 'fit' and popular and just seem so much better than me, I could rant about this myself but I won't xD Although I shouldn't be doing this, like you shouldn't with your sister. Please don't worry about putting weight on, it's healthy, and even when you are craving snacks and you're still paranoid about putting weight on - you could resort to healthy options with low calories, thinks like carrot sticks, it might help you feel more comfortable about food. The number on the scales doesn't determain how physically fit you are, you could weigh 60kg but you're really fit - with a flat tummy. My goal is to become fitter, and get abs and become 'tonned', but I'm really lazy so I don't know. If non of these things are helping, I found this special tea called 'Flat Tummy Tea', I don't know many details about it, but it gives you quick results. -
http://flattummytea.com/ - I hope I kinda helped somehow and made you feel a bit happier, it sucks being lonely. Please just remember that things are going to get better, a lot of people here on tbt will support and help you~