As suggested - here is Lahan's older brother (Junjie) in the young Maomao Lahan siblings AU.
I think he would be the very tired eldest brother (even though they're supposed to be cousins) but he has no choice but to handle these two, and they are definitely a hand full. He can be quite soft on them, since he can't bring himself to be too harsh.
I'd think that Lahan and Maomao would be around 11 and 10, whereas Junjie would be like in his mid-late teens? I don't actually know how ages work.
And yes they all have blue eyes because I've registered that as a La Clan thing (though Luomen has yellow eyes? In the anime).