Why does my little sister do this?

I'm a 13 year old girl and I have two siblings: a 10 year old sister and a 7 year old brother. My family aren't the richest, but my sister certainly doesn't go without and neither do me or my brother. My Mom buys us all stuff when she's out and about, but if she buys me or my brother something, and not my sister, she cries. It really confuses me because when she gets something and I don't, I'm fine. I can understand she may be a bit jealous, but today, she was really pathetic. My Mom bought my brother some sungles for �1 and a video for 50p and some books for me that came up to �3, from a charity shop and my sister started wailing. I thought this was unfair because yesterday she got a �10 dress and neither me or my brother cried. Is it normal for little girls to be like this? I wasn't like it as a child. Thanks in advance.