Only the carrier the phone is/was branded to can unlock it.
If T-Mobile sold the phone, they will unlock it, provided that certain requirements have been met. Essentially, T-Mobile is not in the business of selling phones for everyone to use or export as they see fit. They want to make sure that the phone is paid for and used on their network. If your sister has an active account with T-Mobile, she can insert her SIM in the phone and request an unlock. She can do this up to twice per year for each line of service she has.
I would like to know if you'll still have the galaxy 8 plus on sale