I hope you are feeling a bit better now that you have opened-up on here about how you have been feeling.
My advice would be to seek out a counselor or psychologist to talk through what you have written on here. You could see if your school/or wherever you are have a counselor; or you could book a Doctor apt (just the local GP) and ask for a referral for a bulk-billing psychologist - so there is no cost to you. And you can talk through all these things and they can help you with understanding things.
Also I would forget the facebook - try your best not to look at the photos your Mum puts on there. Instead, try look at things/read things that make you feel better about yourself. There are lots of areas on this website you could read/post in. I am sure you will find lots of others who have similar thoughts.
Your thoughts and struggles are just as important as anyone else's. They are very real to you. But it is probably better to discuss them with a professional - who shouldn't judge - rather than your friends.
My father and brother used to call me fat etc too when I was growing up. It is horrible. It stuck with me and I have always had body image issues. I am now 37 yo and my strongest advice to you would be to not give any worth to what they are saying. And instead give worth to YOU and all you are more than your body. You are a personality, a brain, a friend etc etc. And your body is to function - so look after it and love it for you to be healthy. I have been in hospital before and the nurses say to be your own best friend.
Try your best everyday to be your own best friend. You are worth it :)