Living as brother and sister is advice that is usually given to couples already in an invalid marriage, who have repented, and wish to receive the sacraments, but whose circumstances make living apart impractical (for example, if there are children in the home who would be traumatized by a separation).
I am surprised that a priest suggested that you move in with your BF. For you to do this without benefit of marriage would place you in a near occasion of sin, by making your commitment to future chastity more difficult.** Furthermore, it would certainly be scandalous to those who know you, as most people will assume that you are having relations if you are cohabitating.** Live near each other, if that will help you parent together more effectively, but not with each other.
I would suggest living apart until you marry (if you decide to marry–think about that decision carefully), and getting a second opinion from another priest.