I just don't think that Yu Nuan adoption into the Ling clan will be complete until she has her own powerful, wise, semi-unintentionally murderous mother figure.
Thunder Mom!
Besides, Yu Nuan was complaining that her thunder-based movement technique was too limited because it was lightning dependant. So I think that participating in a 4th realm lighting spirit's game would bring a solution to that problem, rather than doubling down with the thunder spirit she is already close to.