In my opinion, you should be dumped. That's her sister you said that too. If anyone told my sister what you told hers, they would be out the door on their asses so fast they wouldn't know what the hell happened.
Family completely outweighs boyfriends so yeah, if you can't accept her sister the way she is, then you should be dumped.
How does her weight affect you? Her sister has absolutely nothing to do with you so you had no right to say what you did to her.
And FYI, she probably was depressed and crying about it. Girls are very self-conscious about their looks and weight, especially during the teens years. With all those perfect models out there, it makes girls feel like they don't measure up. The more they weigh, the more self-conscious they tend to be. You probably completely killed her self-esteem and you should feel ashamed of yourself.
Also, don't have a girlfriend until you learn how to respect girls. Weight is NOT a topic of discussion.