Purchases from main account transferring to friend/family member's account when you sign in to their account

So I usually get on my brother's PC when he's not on and after I got off, I got on again a couple hours later to play with my friend. I realized that ALL my purchases were gone including my minecoins and have no idea where they went. A couple days later I logged into my brother's account to help him with an issue on my PC and got on minecraft and then HIS purchases transferred onto my account. I have no idea what's happening with minecraft all of a sudden but I really hope this gets fixed and hope we can get our stuff back