It will be much simpler for you if you just sign up and use Yahoo or Gmail - you won't have to mess with Outlook or settings.

You can choose any name for your new email address ( it will let you know if that name is already being used, then you just add something or make a new name till it is accepted), you'll still get your other email how ever you get it now.

I have 5 different email addresses, just because..

About the computer admin account, If you can't sign on to the admin account how are you using it now, as a guest or a limited user?

Those accounts can't do too much as far as major changes, only the admin/owner account can do those.
I just checked and my dh isn't allowed to make a new user, he has a limited account. I'm the admin/owner, main pc fixer person..
Guest account can do even less.

If sis gives you the password then you can use her admin accnt to make a new user account for you to use & I would leave her admin as it is.

If Outlook asks for a log on- she has a password set up for security reasons, I do the same thing - a password to log on to my computer admin accnt & password for my email to open.

Businesses often use Outlook because it has a lot of built in extras for organizing, meetings, and contacts.

[Will it work if I erase my sisters address and put in mine? ]
You can try it and see. It won't break anything.

Oh was it a link to contact the clean air agency??
and that's when Outlook opened up?
If so now I understand.. I thought you were trying to contact the Outlook dept at Microsoft.

Yes a contact us link will pop up the default email program- it may or may not work to send an email with your address.
It depends on if your sister had the same kind of email provider.

example-
A gmail email can't be sent with a yahoo email address as the main one in outlook or any email program