because ...

It is worth noting because there are two chief objections to incest: one is that one party may feel pressured into sex because they are dependent on the other or due to family-related pressures; the other is that potentially sexual relationships (e.g. marriage) between close relatives causes a significant increase in birth defects. For example, marriage between first cousins roughly doubles the birth defect rate to about 6%, and a blood-related niece is more closely related than a cousin. Neither of these concerns applies in this case, and if Rainbow marries his "niece" at a future date there is no need for concern on either of these counts.