What do I do to help Isobel?

My sis Isobel (called multiple names including Maybelle) really wants a DSi. She has about $100 in her bank account (thats what she told me inyway) and wants to trade in her DS so she has a bit of money offf. So mom will have tto pay about $20 id say. NOW she made Isobel sad. she said no. i hate to see my sister sad. what do i do to help her? she told her close friend she was getting one. i wanna help