Just getting to know this girl and she said that to me

Just a guess but it probably means that your closeness with your sister is adorable? Haha it's not that common now to see really close siblings and it is adorable when you find siblings that are close.

It's a complement. She appreciates that you and your sister are close.

That in her mind, siblings don't get along that well, so seeing you with your sister is adorable as in like cool. Because of the fact that you two seem to get along well.

Good Luck,
Javi