How about removing the sidebar on the page that has the login form in the content?
You can also use a plugin like this to show/hide widgets depending on if the user is logged in or not: https://wordpress.org/plugins/display-widgets/
However, that will only work if the login widget isn’t the only widget in the sidebar.
If neither of the above work, you can use a function to set the sidebar layout depending on if the user is logged in or not.
Let me know 🙂