To remove that "family member", select the member and at the bottom, there is an option to remove.
I recommend you first sign out everywhere at the bottom of that page, then change your password. You can find a guide for that here.
As the Premium for Family owner, you will be able to remove "family members" anytime.
If you got a free upgrade, congrats. Setup your family or friends "living at same address" with premium.
After doing that you should be fine. However, if you are still feeling uneasy continue reading.
You should contact customer support as soon as possible to determine what has happened. Use the following link here to access customer support (Subscriptions -> Family -> I Still Need Help). Email with pretty much your exact message above. You'll have to prove who you are then they should be able to tell you when your account upgraded.