My cousin changed now she is being kinda a jerk to me what do i do?

well my cousin lives in phoenix and she came down to flag for 3 weeks. well she trashed my artwork which took hours to finish and perfect she bosses me around and shes only 10 and i got sick and tired of her bossing everyone around so i stood up to her and i get really nasty words from her when i do cuz i stand up to thing that arent right. her older sister 12 stood up for her and blamed everything on me. my buddy who is also my cousin10 came running out crying we talked for 4 hours and just cried and she told me that my cousins been talking about me behind my back though im a sweet girl so i cant talk bad or curse or talk behind peoples backs. well the 12 yr old sister came out and we were still talking and i said at least i dont talk about people behind there backs oh wait nother mind its always my fa uld. today i took the blame for something my buddy did and i stood up for wat i believed in again. well everyone was tooken away from my buddy and i. some how this morning i had my art work and it was wet from glitter glue and then all of a sudden later in the day it dissappeares. where? i dont know i looked everywhere. well my cousin said well someone could have thrown it away but look i made and extra i said no its not the same so you thrown it away. good job. now i have til monday to get it all done again and thats gonna be really hard. what do i do to get her to leave my stuff alone stop bossing me around and be nice?