Mirror Legacy is indeed about the legacy and heritage of this family through the guidance of the Mirror. All of the brothers will eventually die, their children will continue their legacy and eventually die as well. With each member dying the Mirror absorbs their cultivation and techniques making the next generation stronger.
There are 2 protagonists in this Manhwa:
One is the Mirror which is the traditional protagonist but also the passive one most of the time, he can help and direct the family but it will take a long time before he can do stuff on his own.
The second protagonist is actually the Family as a whole, if you remember thir father mentioned a Family poem to be used to name their descendants for each generation, the 4 brothers here are the 1st generation and currently the Light novel is at the 7th generation.
