Why is it exactly that you want them to sleep apart? Do you and your husband sleep separately, too? It's perfectly natural for humans, like many creatures, to want to sleep together. My two cats (brother and sister) cuddle up and sleep together all day long, for example, and it is sooo adorable.
The only reason I can come up with for this strange taboo we have in this culture for sleeping as a family is that some people must consider sleeping to be a sexual act, but good gollies, sleeping is just sleeping.
My advice is that it will be time for the kids to sleep apart when *they* decide it's time to sleep apart, and in the meantime, for you to feel proud and relieved that your children love one another and are creating the foundation for a strong, committed, and nurturing relationship that will hopefully last them a lifetime.
~K. (who wishes she had had the opportunity to get to know her own siblings in this way while growing up, because we might have more than a xmas-card relationship now!!)