My advice would be to choose someone you look up to and who will mentor you as you grow nearer to becoming a sister yourself rather than someone who is just a friend. If that person happens to be a senior that's fine.
I joined as a Junior too and my big was also a Junior. She wasn't that involved her senior year so it was kind of as if she had already graduated. But, 20 years later, we're still very close. (We talked on the phone just 2 days ago.
)