Unanauthorized Trade-In from In-Store Rep??

So I switched from T-Mobile to another provider a little over a month ago. I was the primary holder of the account but kept the account open since my brother and sister were on my plan. We were going between each other to decide who takes over account ownership or if they open a new account since my brother's credit is fixed. Long story short, I got an email saying they haven't paid any services nor have they sent me money to pay the bill as guest now that I no longer have my T-Mobile online account since porting, so I followed up.

My brother apparently went in person to a store (this store used to be a Sprint corporate store before the buyout and I think they became a T-Mobile authorized retailer) and they told him they couldn't handle an account transfer. (The is the exact same Sprint store I had my account created at through an account transfer in Dec. 2019. Just my luck.) The rep in said store told my brother he could trade in his and my sister's phones to open up about new account- phones financed under my credit and name. I technically owned these phones, not them. My brother is only authorized to make payments. I have driven to this store twice in previous times because I had to be there with my license to sign the paperwork and approve the upgrades for both siblings. I also had to be present when upgrading my roommate's phone during the time she was on my plan. I know for a fact that I'm the only one allowed to make any changes to financing after doing this for every other line that wasn't mine in the 7 years of owning the account.

Without calling or sending a text to me, this rep allowed my brother to get whatever promo they offered, using devices that were financed (and paid off) under my name and credit. I'm not mad at my brother and sister for getting off my plan and trading in their devices because I told them that they should do what's easiest for them. What am I absolutely livid about is how this T-Mobile rep guided a person who was only authorized to make online payments and view the account into trading devices from an account they owned so that could get the commission for two new lines.

I'm calling to cancel T-Mobile now that I know my brother has been getting service through them this whole time. Is it worth noting that non-authorized trade-ins occurred to open a new line? And if I were to report what I believe is fraud, would it affect my brother's account if they were to investigate?