How can I get my favorite possession back from my friend?

ok so my bestie and I started to write poems a while ago and we decided to make a poem book and write all of our poems in them and let each other read them so she wrote a poem in it and gave it to me so then I wrote a poem in it and showed her I kept it for a while and then she asked to borrow it and I said ok but please dont lose it ok and she was like yeh ok and now she tells me that her sister stole it and won’t give it back and im really upset because it had one of my fav poems in it that meant a lot to me and I cant remember how it goes..so ill never here that poem again..but what really makes me angry is that her sis could be up at her place tellin all her frnds that she wrote the poem but really I wrote it..or ma frnd could be lieing and didnt lose it and jst told me that so she could tell her frnds that she wrote em..I know she is ma bestie but she has lied to me before..what should I do?