I'm 13 and my mom is annoying me?

Well, the question may be much different from the title, but bear with me here. I am 13. I have a sister Isobel/Maybelle who is 11 years old. I stick up for my sister because what is not to love about her? You see, Isobel really wants a DSi. She has one hundred dollars in her bank account (well I think anyway. That is what she told me.) and she wants to trade in her DS. My mom had a bit of thinking to do, but after a while she said "It's not your birthday." But izzy promised to pay! Now mom is annoyed and will not hear any more about DSis. I feel really bad for her. WHAT DO I SAY TO MY MOM TO HELP IZZY? Because now she's sad because she told all her friends she's getting a new DSi apparently. Poor Izzy...