Please help me with my sibling trouble!?

I am the oldest of 4 children. I am 13 and I have 2 younger brothers, 11 and 6, and 1 younger sister, 8. I feel like they gang-up on me all of the time. Just really discreet things like sneaking into my room and hiding plastic toys in my bed, or hiding my things, or even just calling me names and saying horrible things to me. There are a lot more things that they do, all small, but I'm quite hurt by them. They don't listen to me and they mock me all of the time. I feel like I'm keeping too much inside. Both my mum and dad were the youngest in their family, so I don't think they understand. I know it's not the most important thing in the world and I know it sounds silly and I know they are only little but I am quite intimidated by them, it's probably because there are three of them. Does anyone have any ideas on how to deal with this problem? Thanks in advance.