Just a couple ideas. I take it this is a double bed? I've noticed my game assigns sides of the bed, Seth gets the left and Christal gets the right, or vice versa. My married sims who have a good relationship have no trouble switching if their partner has already occupied the wrong side of the bed or if something is in their way there, but I think that's why you may be getting the Sleep in _____'s bed and it's the wrong person. If it's a single bed, might you be changing "ownership" of the bed if they're switching off sharing it?
If it's a double bed, do Seth and Christal have a good relationship? If not, they won't share a bed.
Do they act like they don't want to get in bed? That would indicate the above, that the person sleeping there already is someone they don't want to sleep next to. Or do they act like they want to but can't? That would indicate something is in their way. It's worth trying to move the bed. I would try moving it to a different room and moving it outside and asking them to use it and see what happens. Sometimes particular rooms get stuck with problems like that.
I would just buy a new bed, though. I know it's expensive if the family is low on simoleons, but if the problem is the bed, that would solve it and it probably won't happen again.
