It's activation skill overkill at that point.
I look at my Noire in my Lunatic play through. She is a Bow Knight with Avatar as her daddy. And with her capped skill 46 (44 with +2 Skill) she inherited Vengeance from Tharja (should lved her more and have gone for tombbreaker) and Ignis from Avatar. The problem in this scenario is that Vengeance has a higher proc chance and would over right Ignis which in most situations is better than Vengeance. So I just took off Vengeance. The lesson here is to just choose only one activation skill.
In your case one does more damage and the other more healing, while the last is for killing targets that would otherwise be unkillable. In most cases (specially harder difficulties) self healing (Sol = better healing, Aether = better damage) trumps higher damage (Ignis/Astra and Astra is a /die against Counter targets).
So as a child character Lucina's and Morgan's str would be 52 and 60 skill as an Assassin after Limit Breaker. Which means pretty much guarenteed skill activations, Morgan even more so with Rightful King. Lethality loses its flare now that they have high enough str to kill most targets (and that most bosses in later diffculties are immune to it anyways). Sol would be the best choice for self healing, Astra or Ignis for pure damage, and Aether for a nice inbetween.







