no it is not a sin unless one of you sleeps in somebody else’s bed. What I would argue is wrong if one parent routinely (not with a nursing mother or sick child, but routine) sleeps with one of the children, that is skewing the natural order too much. In my parents day couples usually had twin beds in the same room, so mattress, covers etc could be customized, but yes with a snorer who refuses to get help, for instance, separate rooms may be necessary for healthful sleep. an even better solution is to deal with sleep issues and make the bedroom the place only for sleep and romance, and remove all other distractions–electronics TV esp. being no.1, also laundry to be sorted, kids toys, books and everything else that is not related to those 2 purposes.