My cousin puts me down a lot?

Okay so let's call my cousin Mya. She is really rude to adults but claims that people love her. We both go to same school now but before we were in different schls. She's always trying tonne popular and always tells me she is. I'm not popular but I'm definitely not unpopular. In my new school my group of friends is kinda the popular group (because two ppl have told us that already). I feel like we're labelled. Lots of people have told me how Mya is really rude and so many people don't like her.

If I'm calling someone else and she sees me online she will not stop bugging me till I cut the call. Whenever she comes to my house (not of my will, our mums are sisters and they are so close) she sees my phone and picks it up and casually goes on my whatsapp. When I tell her to get off she just rolled her eyes. Once, I got a brand new Samsung and she had her old iPhone 4. We were making paper Mache and she thought i wasn't looking and put my phone on the glue making it all sticky.
She accuses me of copying her. She got the game colour switch which is my fav game and apparently I copy her. I love watching seven super girls and she said I copied her.By watching them. Whenever I go to her house she refused to give me her Wi-Fi password and sits on her laptop, online shopping, with headphones, completely ignoring me! What to do

Treat her the same. Don't go to her house. Put your phone away or stay on it with headphones and don't acknowledge her.

Have you ever sat down with your mum and discussed this? I don't mean ranting when she does something rude, or whining that you don't like her. I mean an "adult" type conversation, where you calmly lay out why you have problems with her and, more importantly, what others think of her.

So be careful what you use as examples. If your mum is sane, she's going to pay a lot more attention to the things your cuz does at school than the conflicts the 2 of you have individually. From an adult standpoint, the online and Wi-Fi stuff you mentioned sounds petty. Focus on how others perceive her, because this means she can't blame you for anything.

If it was me, I'd say something like "Mum, I know you love Aunt X and Cousin Y, and for that reason, I've tried defending her. But I can't do it anymore, because she's so mean and unpleasant I look like a fool. I'm starting to make friends with some really nice girls, but they don't want any part of her, and I don't blame them. If she wasn't my cousin, I wouldn't want any part of her, either".

Just talk to your aunt about her, maybe she can help you deal with her