RANT about high school kids bullying my sister

I'm tired of the way society views beauty. My sister gets made fun of for not being what others view as "beautiful." She's thick and she's happy with her body. But somebody at school always brings her down for it. Today her boyfriend was arguing with her about how kids at school make fun of them because she's bigger than him. Instead of defending her or making her feel better, he asked her to change her image and get on a diet. He tells her he's tired of his friends and people on their bus and at school picking on him about their weight difference and he wants her to do something about it. My sister just broke up wit him and came to my room really sad about all this. I'm tired of seeing my sister hurt over this. Because she doesn't have a 28" waist and isn't "slim thick" she's viewed as this ugly person for her weight. I know my sister is beautiful and loves her body the way it is but I can tell that this is starting to tear down her self esteem. I want to help her and show her that the opinions of others should not have any affect in her life. I don't understand why my sisters body or the way she views herself as beautiful is a problem for others.