First of all welcome to the HW Depression Forum, you have come to the right place here as we are all suffering depression for whatever reason and we all try to help one another.
You know I am the black sheep in my family - I'm not saying you are the black sheep in your family, but I am just making a comparison. I have three sisters of which I am the second eldest. I am the big girl of the family, the one with the big build, the big feet, always left out of things, always felt I wasn't worth any boy looking at me twice when I was younger, always had to compete harder to get that promotion at work because I was the big girl, never had a lot of friends at school or even now. It all comes back to self esteem and I am now 56 years of age and I still have low esteem. But I know I am a good person, I have been a great mother, I am loving, caring, compassionate, I have a lot of empathy for people less fortunate than me but most of all "I love me
It is pretty normal to resent your sisters, I did mine. I think it seems like they are picking on you because you are the youngest. I think maybe they should include you more in their lives, encourage you to lose a little weight, go shopping with you, experiment with makeup and hair colours etc, things that sisters do, maybe all three of you could join the local gym or go walking each evening after dinner, something that you can do together, maybe you should suggest this.
Why don't you see if you could do some deportment classes or self esteem courses or even speak to a counsellor? All or either of these would help you. You sound like you have some anxiety as well, you may even need to or feel better if you speak to your doctor, tell him what is happening with you and how you feel. He may suggest counselling or even some of the courses that I have suggested.
Remember you are most welcome to post here and I hope you will continue to post to let us know how you are going. I hope I have been of some help.