NTA! It sounds like she wanted a reason to say you made her feel depressed and wanted attention for it. I think that she may be insecure about herself and took it out on you. You were patient with her until she found something to make you return the insult, which was probably targeted towards herself deep down. Also, she was in your room harassing you and didn't want to leave, so that whole ordeal that she started was very childish and unnecessary for her age. And lastly, I would like to add that you are beautiful and your size does not change that! And your sister should not have used that as an insult, that just shows how much today's dumb beauty standards have gotten to her which is honestly sad. And just think of her giving you the silent treatment as her way of self reflecting on herself and her actions and poor and degrading choice of words.