Yes, Luna is my younger sister, and we have ruled Equestria together for quite some time. [That is a half-truth, maybe three-quarters when you consider that the period of time Luna and Celestia ruled together was indeed quite some time as a human would reckon it.]

The title of 'Princess' is a complicated one, but in modern times it refers to alicorns -- ponies, like myself and my sister, who have both wings and a horn. In these days, that privilege is earned, not one of birth or lineage.