Why doesn’t sunny spread the love of shadow serpent chap 2000+ spoiler

So I’m on chap 2050 and sunny has already given the mark of the shadow to his sister and aiko. This obviously expands his domain and it buffs them both. Why doesn’t he give this to all of the gang like neph and the fire keepers. Did they explain why he can’t give it to neph at least cause she would definitely agree.