You make yourself "nationally known" by participating in events and presenting your organization in a good light. Like LatinaAlumna said, though, you can't become a national organization unless there is interest at other universities.
Looks like you just became a member of your organization. Enjoy your time as a new member before you worry about trying to make huge changes. Your chapter's leaders may already have a plan in place to expand.
And, btw,
talk to your sister.