How can I ask my mom without being spoiled..?

So my graduation is coming up in A WEEK and i dont have a dress yet, technically. When my older sister bought her dress for her graduation my mom expected me to wear it too, so she wouldnt have to spend money on two dress (were not billionaires....) I mean I understand what my mom is saying but I hate my sister's dress! Its not me at all! all pink and flowery......:( How embarrasing! My mom hasnt even said anything about buying a dress so im sure she doesnt want to..... I havent said anything either because I thought she would know and i feel spoiled asking her and such a burden....