There were 2 hot light and 1 cold light. On a meta level, Doug confirms one of my guesses - the interpretation should be that Etsuko, being female, is Yin, the cold light.
So, she gets "consumed". However, it may also mean that she becomes the adviser to her brother, but never officially holding the title, since the light was not "snuffed out".
Being conservative and with references to feudal Japan means that Etsuko's position is actually quite strong. As the eldest daughter and being upper caste means that she is expected to be able to support her family in roles greater than home-maker. This, plus a world female samurai is literally accepted and is norm, should place females in the L5R world in a stronger position than real-life Nippon.
Estuko is likely capable and makes a good candidate. Being personally skilled in the ways of the courtesan is a bonus. Both she and Mariko has their own secrets to guard. Deadly secrets if used wisely against them - more so for Etsuko than Mariko. Of course, Tanabi will keep his silence on this hint (at least, for now). Need to know more about her. She will get married, and depending on the rank of our mystery groom, may as possibly she marry away as he marry in.
Seiji has only one claim: that he is the eldest son. This is a weak claim, since if it bore any real weight, the PCs would not be called and he would be ceremonially appointed as next in line already. Plus, unless he reveals more character than current, seem to have the least capability as well. He may likely server best in position of Military adviser.
Initially, I had place Seiji as the aggressive light, but I'm wondering if it is not Mariko who may be planning a coup; being, on the obvious face (at least), apparently the least likely candidate. He is seems ambitious, intelligent, and very willing to challenge customs and traditions. Great attributes, but also potentially dangerous ones (to himself and others), if not checked by the wisdom of experience. He keeps his quiet counsel for the moment. But he has friends which we should get to know more about as well to see the real extent of this capabilities - both in defense and to also consider this as a point for putting a vote in his favor, or against.