In the manga, it was implicitly hinted that Road and Neah have a connection - a deep relationship. (That relationship subsequently spilling over to Road’s current attraction and fondness for Allen Walker - Neah’s host.)
What if Neah sired a child with Road and that child was Tyki? It’d explain how close Road and Tyki are as compared to the rest of the family (exception of Jasdebi); and how Tyki resembles Neah in terms of appearances…
It’d be the reason why Neah left Road unharmed and why the Earl didn’t prosecute her for helping Neah, because she was pregnant. During the fight in the ark, Road was positively pissed over Tyki’s grave injuries (and only Tyki’s) despite it being Allen Walker the one dealing the mortal blow - because it was her child he was hurting.
Not forgetting that adult Neah was alive 17 years ago, Road being the eldest Noah who survived the massacre, and Tyki currently being 26 - the timeline fits.
Maybe that was why Road silenced Wisely when he pointed out Tyki being the splitting image of Neah - to conceal the truth from the rest who had their memories damaged.