Lapsed,
I’m awaiting the convalidation of my marriage. It’s a first attempt at marriage for both my husband and I.
It took some trial and error to figure out how we ought to be living during this time period (which for private reasons has been ten months long and is expected to continue for another 2 or 3 months). He had to sleep in a different room at first, but now that we have gotten used to the idea of the situation and mastered more self-control neither one of us finds it problematic (read: near occasion of sin) to sleep in the same bed.
As Celia said, we behave as a chaste dating couple should, the only difference being that we live together (it would frankly cause more scandal and problems if we were to live separately).
Here is my understanding of what it means to live as brother and sister. You don’t have to feign disinterest in your spouse during this time or act like a literal brother or sister. “As brother and sister” is just a euphamistic way of saying “abstaining from sexual activities.” Each spouse should cherish the other during this difficult time and take whatever measures necessary to avoid placing his beloved in a near occasion of sin. If one spouse does not understand or esteem chastity, the other spouse must do her best to guide him. The specifics of this will vary from couple to couple.