Managing personal Outlook.com account settings, security, and privacy
Hi. I'm Brian and I'll try to help.
Explain in detail exactly how you tried to create an account for your family member. If you were actually creating a new account, you would not be having this trouble. However, if you actually created a new alias for your own account and then removed your original primary alias, your symptoms are exactly what you would see. This is why I ask what steps you took.