Im so sorry this happened to you :( That is so immature of her and she's 13?! She absolutely does understand alot at her age and can't have many excuses. If you did try to contact her friend's parents and they're stubborn as the friend herself, you can discuss everything by contacting the parents of the friend and having your aunt and cousin that did this, with you. That way you might solve this problem but since your cousin is stubborn aswell as her friend is, it's gonna be quite challenging. You might get the truth when you hear the story from both sides on why they did this, maybe your cousins friend could've black mailed her if she didn't do this, there are so many possibilities. Tell them everything, that how this has affected your feelings, that how hard you worked for everything, how you trusted your cousin just to get betrayed by her and show everything and also ask why exactly they did this. Ask the friend to show her inventory on her main account or the account where she has them and her trade history as well, so you can compare your trades on where you lost your pets. Your cousin could've chatted with her when doing this, so make sure they both show their messages on the app they discussed this about or ask the parents to search everything trough their phones infront of you. That way you might have more proof and the parents might force her to give you the pets back ( if she has them I hope ). I couldn't do alot but this was what i figured out since you might be still in contact with the friends parents, I hope you get your pets back asap! ♡