.

They're not close, emotionally or physically, they haven't seen each other in a long while at the beginning of their cooperative work. Whatever coordination of minds they have going on is explained in the literal plot: they cooperate well, they work together well despite their one-sided rivalry. They're equal and surpassing L when they're united. There's no basis for the "sibling coded" relationship because, for all it matters, they have not been in proximity to each other after Mello left the orphanage. There's little to no detail on how they spent time prior to that, but Mello is shown being close to Matt, not Near.

They have never referred to each other as "brothers" or anything similar, never displayed this "familial" bond that can be seen between real siblings within the same media piece.

Not to mention absolutely zero blood connection between them.