In my experience if they are brought up together, a ewe and a promptly castrated wether will rub along just fine, with no hankypanky nor aggression problems. This is for whiteface Dartmoor; wouldn't wish to speak for other breeds in the same situation, but doubt it would be different. Incest is a natural taboo, but through segregation and standard stock-keeping methods we remove the relationship continuity and allow it to happen sometimes. The downside is that when you lose one, the other won't be far behind.