If you are not a member, you need an invitation first. If your family member is still a member, they can invite you by making you a profile with your name and email address. Nothing more is needed.
After you've accepted the invitattion and signed in, they can then make you the manager of all the profiles using the Bulk Profile Manager Changes tool.
If your family member has left WikiTree and the profiles are "orphan" profiles, you'll need someone else to invite you. The people to ask are here. Then you can "adopt" all the orphan profiles.