Bra help...crazy mom...?

Ok, So i am 16 and i have been wearing training bras since i was in 6th grade because of the whole gym thing. Talking to my mom and telling her I needed one was one of the worst experiences of my life. And like I'm a teenager so i wear v-neck shirts and tight shirts so I have to wear the training bra under it. My mom always makes fun of me about how I have nothing and how flat chested I am. We are not close at all which makes everything so hard. And then I have an older sister(just a year difference) who thinks she's so much better than me because she wears real bras. But I checked and she only wears 32A which is really small and sometimes she wears padded ones, and I got curious so I tried on some of hers when she wasn't around and they fit perfectly! Like sometimes I'll wear cami's with the built in shelf thing but it's really obvious that I don't wear a bra and sometimes my show but my mom does nothing except make fun of my small chest.But I know that I'm a 32A and sometimes I'll even steal my sister's to wear and no one notices the slight difference. It's very embarring that I haven't started growing because I am 16. It's summer and I really need bras to wear under stuff like summer dress and tank tops. My mom just makes fun of me about everything and refuses to buy me anything,even new panties and socks!!! How do I talk to her? And I would ask my sister but she makes fun of me even more! Advice on how to talk to my mom?!?!? I don't even know how to bring up the conversation of bras. My mom doesn't even like buying me new training bras so I wear the same ones over and over again until it actually rips and I have none left. Right now I have 2 but you can see that I'm starting to grow and it is so obvious and I know guys are noticing too! Please help!