I always wondered what caused the difference in eye color. Magic trait? Are they actually fraternal twins, against all odds? Is one of them wearing contacts?
It could also be another case of heredity, especially Mendelian inheritance. Yellow/Blue-eye genes from their grandparents may either be dominant or recessive; should, say, the yellow-eye gene be recessive, it would still be passed onto their father but remain invisible phenotypically (i.e., his eyes are blue because that's the dominant gene). As he carries recessive yellow-eye traits, there is a low chance that his offspring will "suddenly" have yellow eyes.
That would fall under "they are actually fraternal twins, against all odds". What's specifically throwing Slave off is that the two sisters are totally identical in every way except eye color - if they were identical twins, Mendelian inheritance couldn't explain why their eyes are different colors because they would have by definition inherited the exact same genes from their parents. However, if they aren't identical twins, then the odds of everything else other than the eyes just happening to turn out the same would be astronomically low - not impossible of course, but unlikely enough that them being identical twins with some non-genetic explanation for their difference in eye color might actually be the more probable explanation.
That would fall under "they are actually fraternal twins, against all odds". What's specifically throwing Slave off is that the two sisters are totally identical in every way except eye color - if they were identical twins, Mendelian inheritance couldn't explain why their eyes are different colors because they would have by definition inherited the exact same genes from their parents. However, if they aren't identical twins, then the odds of everything else other than the eyes just happening to turn out the same would be astronomically low - not impossible of course, but unlikely enough that them being identical twins with some non-genetic explanation for their difference in eye color might actually be the more probable explanation.
Epigenetics (different expressions of genes because of environment). Or some somatic (post-conception) mutation.
Eye color is one of those complicated traits that isn't strictly Mendelian. Multiple genes determine eye color, and changes in melanin content over time can cause irises to darken or lighten with age. Injury can also cause eye color change up to including heterochromia.
A famous recent case are the True Blue Twins: one of them has two blue eyes while the other has one blue and one dark brown (heterochromia). Another case happened in the UK which was particularly notable because both twins had drastically different eye and skin color at birth despite having the same DNA.
