So my sister promised me a late christmas gift..?

She told me her bf's mom was buying it if they could get $20 to pay her back. After about an hour of bugging her to tell me what it was, she did. It was an autographed MCR poster. After I got extremely light headed and almost ped out she told me there was a 98% chance of them buying it. She always promises me things that she never gets...So yea she has 2% to fall back on. Its past national killjoy day. Its almost new years.. So yea I'm kinda mad since my sister promises me things and then says "IF". Any adivce? Other then to stop exspecting my sister to keep a promise..