Mean Cousin or Jealous?

My cousin ls 10 and she wanted a 3ds but i asked my mom to get me one in front of her since she was getting ready for work and she mumbled "stupid rich girl" as few days past i got a blue 3ds that i had to work for and i was at my aunt place when she saw me with it in my hands and started to get really angry and started tried to punch and kick me but i learned self defense in p.e so it didn't work and she yelled out she always get what she wants and started cussing but her father saw and took her home At the next day i was chargeing it and i was sleeping but as i woke up i notice my 3ds gone and i ask my sister if she seen it she said yes and grab her cousin bag and took it out but she saw her took it out her bag and tried to slap her but i was with my sister and took the hit but my aunt saw and called her parents and mine. when my parents came they talk to her and she left but im scared now because i have a family picnic at this beach and shes going too but other family relatives are going what should i do to keep her not violence when her parents are not looking?