In my family, we're two boys, one girl. At various points, my brother and I shared a room either by choice or by circumstance, but my sister always had her own room after she graduated from the crib-by-the-parents'-bed stage. Of course, kinda hard to say what was avoided since the alternative never occurred. At the same time, we didn't feel cramped or deprived growing up and actually I think my sister may have wished to be in the room for awhile.
If they're rather young, I see no reason to separate them at all and I don't think that would really be a problem up until at least middle school, maybe even early high school. However, if they're 4+ years apart, then the younger one will get her/his own room much earlier when the older one starts hitting those early teen years. Disregarding the trend for puberty to come earlier and earlier every generation, the older kid will likely be at a time where they're getting an increase in workload out of school, maybe a job or two, more personal responsibilities, etc. while the younger one still has the carefree life of a child. Of course, no one can know beforehand how everyone will react to the situation, but giving them separate rooms at that point if possible is probably prudent to help develop some independence. As for the incest taboo, modern society has turned up some strange characters. Paradoxically, separate rooms might actually facilitate it since room doors would probably be normally closed more often than with a shared room.