That's not a Tia! (moonfire spoilers)

[Anvaire]

We assume that R'fuhl is a seeker of the sun, and indeed he may be as he lacks the seeker of the Moon fangs. Which could suggest that the Nuhn for his sister differs from the Nuhn who fathered him.

However, the same cannot be said for his sister, she had both the fangs of the Moon Seekers and the eyes of the Sun Seekers.
This suggests, at least to me, that she is perhaps half moon, half sun. This would certainly explain why they appear to be outside of the usual tribe conventions. But, it could also be just poor design on square enix' side.