Sometimes I feel like telling my sister off!?

My sister recently got engaged and is planning to have her wedding by the end of the year. She wants my other sisters and me to be her maids of honor. I have no idea why since she had told us multiple times that she didn't even want us to be bridesmaids. We've always felt like she's never wanted to be part of our family. She's 3 years older than me and i've always felt like the older sister with her. She doesn't try to make it to family events or even just hang out. I did live with her 3 yrs ago for 2 years b/c of school. She was a HORRIBLE roommate. I would always be left to clean up after her mess, and pay bills. She would say she'd pay me back, but she never would. She would do this to my parents too. Well now with the whole wedding she is acting like she is this awesome sister and daughter who has it all together. She is so two-faced though, she told me she didn't want to invite some of my aunts because of their kids and that she wanted me to pay for her dj. She didn't even ask me. I feel like telling her to shove it and giving her a black eye!!! I want to tell her off so bad, but more than the anger i feel toward her, i feel sad for her. She doesn't know our family like I do. She doesn't realize how imporant family is to us, and i think that's why it's so easy for her to take advantage of us. How can I tell her that she's being an ungrateful *itch without being one?