MY SISTER IN LAW IS DRIVING ME CRAZY

I'm engaged to the youngest of 9, his oldest brother is almost 30 and his wife is 23. I'm 18 years old and am going to EMT school and nursing school and I am very dedicated. My new 23-year-old sister in law has decided to do the same thing and go to EMT school and Nursing school and copy's almost everything I do, and treats my new brother in law very badly and he does everything he can for her. My mom also saved up a lot of money to buy me a Louis Vuitton bag for my 18th birthday and my sister  called it ugly, knowing how hard my mom worked to give it to me and knowing how much I love the bag. How do I tell her to back off and be original? I mean she's married and 23 years old with a vet assistant license why does she need to do everything I do and then tear me down?