1.- Create a "New Page" (not post) and give it a name, then put the information about Sister 1.
2.- Create a "New Page" (not post) and give it a name, then put the information about Sister 1.
3.- You already have a user account, the other sister will have to be invited as an "Author". In order to do this, go to the Users widget and click on "Add User", add the email address, and choose the type of user. As she is going to post, you can decide to give Administrative, Editing or just Author privileges, then send the email.
On her email, click on step one, which will take you to the WordPress site, where the "user" can be created (no need to make another blog). Once the user is created, go back to the email and click on the button on step two.
The privileges can be modified if needed.
There will be only one blogroll, two author pages and two users.
Something that you can do is go to the widgets area and activate a "Users" widget on your sidebar that can show each users posts.