Why do my parents favour my little brother?

We went to the shops the other day and my brother asked if he could get a video game. So my dad asked me if i could buy it for him. I said yes but only if he would pay me back. It is three months later and I am still awaiting payment. My brother went whingeing to my mum that he didn't want to pay me and then my mum said that she would pay me half of the price back. and i ask my dad if he could pay me the rest but he then said that my brother doesn't owe me anything! please help me.