What does it mean to "live as brother and sister"?

What does the Church mean when it says that remarried Catholics must live as brother and sister in order to receive Communion?

It means that those Catholics living in an irregular marriage but who have grave reason for not separating (e.g., children, awaiting a final decision on an annulment) must not engage in marital relations or in any romantic relationship at all with the putative spouse. They live together as would an adult brother and sister sharing a common household.

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit www.catholic.com.