I love my little cousins dearly but they go through my stuff all the time?

I am 15 years old and i have two little cousins who i see maybe every 3 months. they are 13 and 11 I guess you can say they look up to me and since i don't see them that often i try to enjoy the time we have together. But they go trough my stuff. for example this summer we shared a room and my stuff was all mest up but i thought it was just me. then one day my mom put fake nails on us ( shes a nail tech) and my 13 year old cousin had purple sparkles on her tips . the next morning as i was getting dressed i went to the bottom of my big duffel ( witch i always keep shut )beg to get a top i wanted to wear i found a fake nail with purple sparkly tips . it was way at the bottom of my bag and the only way it could get there is if she was going through my bag. The situation gets worse since this happens at my grandmas house and me and my mom and brother are escaping my abusive dad so we are moving into a suit in my grandmas basement where my little cousins can go while im at school and go through my entire room of stuff instead of just my duffel bag. I dint have anything to hide from them i just would appreciate if they didn't dig through my stuff. how can i get them to stop since they Deny going through my stuff?