Why do people always belittle me?

well i don't know why but for some reason my sister , my cousin , and my boyfriend all belittle me and make me feel small. I really hate it , like for instance my sister , she's always making fun of me and saying stuff about my teeth and my nose , first off i could say way meaner things about her but i don't because i'm not rude like that and I know how it hurts alot. I've already had braces but ive always had large teeth .and my nose is kinda my worst thing to deal with she said it looks like a witch nose , as if i didn't know that but of course she had to blast it out in front of my boyfriend out of no where.And my cousin whenever she comes over she's all nice to me and saying stuff like "oh i miss you " and "dang finally i see you" and or giving me advice or something but as soon as my boyfriend comes around she starts talking about the secrets that i didn't want anybody to find out about and trusted her to keep like, when we were sitting in my room one day she randomly started talking about how i used to wear extra padded bras and i had no and all the stuff i especially didn't want him knowing. yes i'll admit im a late bloomer but i'm more comfortable now then i was back then or at least im starting to feel comfortable in my own skin i'm only 16 years old , but she really humiliated me in front of him but i still say nothing about her just like i always do i keep my thoughts in my head because i have more respect then that. and lastly my boyfriend of 1 1/2 year{s} is always making me feel stupid when were around friends, my family especially, and in general really , today my sister had some enchiladas n i always play around like i'm going to eat it or something really weird like that and he's like " don't give her a bite your don't know where her mouth has been , or like her mouth is nasty EW don't let her eat your food her mouth is disgusting. my sister completely looked at me and thought the wrong way . he's always doing this making me feel dumb and stay quiet , ive asked him over and over to please stop it hurts my feelings but he still does , i'm really getting frustrated and about to just change and be rude . also today both my sister her boyfriend and my boyfriend did that all at once right after he said that she made a comment on my teeth , i'm very self conscious about myself there not helping why do they do this ? how can i get them to stop once and for all? i need really good advice before i go do something i'll regret later please help me : (