[Oh come on. You Egyptians sometimes married your own sisters.]

Bio-science was rather lacking in those times, but there are physical responses that explain the attraction individuals feel for one another. Oxytocin plays an important role in a variety of bonding experiences, including more intimate ones.

[It helps that Ravine has been around for the last few thousands of years to watch science develop so she can know these things. Even if it doesn't apply to her in any practical sense, it's still fascinating to her.]