Sis i feel bad for you..
You know il let you in on something, my dad used to take things away from me if he thought i wasnt doing something right like taking the laundry. Might sound crazy but i used to watch my father whenever he would take things, and see what room it goes into, like a spy. After he left for work i would go through any place in the room i havent looked at. Be smart sis, figure out where it is, 95% of the time when i wanted to find where my dad hid something i found out however it did take some time. Think about the possible places it might be, it sounds childish i know but my older brother even does this lol so no shame. I know you can find it , just think hard, instead of thinking about it take action on getting it back, the diary is yours and no one has the right to read it except you.
edit: Use your mother against herself, ask your mom that you want to see the diary because you wrote something that she needs to see (not really) , then watch which room she goes to but dont make it obvious that your watching her, then when she hands you it just flip through the pages acting like there really is something and then just say "nevermind i forget where i wrote it" . Then give it back (personally i would have just ran off with it if she gave all the diaries and then explain later why you ran off with em and hid , and then explain later).
Usually people only have one hiding spot, if you can figure out the area, you definately have a way bigger chance of finding the thing your looking for