I am trying to help my sister set up a web hosting account with Yahoo. She has already purchased a domain name and she wants ME to be able to webmaster it. How can she set me up to be able to access the file manager to build her site? Any help is apprecciated... :)

If she's paying a monthly fee, then she should have access to an "Administrative Console" of some sort where she can add another ID into the system for you. If she is technically challenged, have her simply call Yahoo support. If she is using the FREE yahoo account, then you have no options here.

She can give you her Yahoo! login and password.

She can give you her webhosting account login and password