Thank you for posting and welcome to the Community!
Being able to activate additional lines on your account shouldn't be a complicated process.
Could you tell us more about the issues you encountered? We'd like to help.
The first line of a Share Everything plan must have a smartphone device on a Premium Tab, Smart Tab, or No Tab payment option. You may add devices to your plan and it should not be problematic. Additional lines can be used with smartphones, tablets, sticks, hubs and basic (talk-and-text) phones, or with Wireless Home Phone (if available in your area).
Every line on the plan gets access to the unlimited features (calling & messaging).
There is a separate agreement (term) to fulfill for each device in the Share Everything plan. The agreement is based on the price plan and term each user has chosen.
If she already has an existing phone & plan which is completely separate, no there would not necessarily be an easy way to do it online.
(especially since it can be under her name, etc.. as not under your account)
All and all, best bet would be to call in and they should be able to get it sorted out.
As they could verify both account holders.