Hi Brandy,
1. Arthur’s WikiTreeID is Rowlen-23. You can start at your Mom’s existing page, click Edit (near the top), click Add Father (under Edit Family section). Then select “Connect an existing profile as her father”.
Enter Arthurs ID (Rowlen-23) then Continue.
2. To add new pages for her siblings you can start at Arthur’s page, Edit, add/remove child, Create a new profile, then continue.
There are help pages but they are sparse: Adding Family