
Yes he’s met Yukio and his mom (Tsubaki) before, when he bumped into his half brother (Ryuuji) by accident. And he was there for his half brother’s funeral. And I think he even kept in contact with Yukio’s mom after she left town, but not sure about that last one.
I’m pretty sure he knows exactly who Wang is talking about when he mentions Yuki, his mom and his half brother that Wang killed, that’s why he also wants to destroy the Heilong (Wang’s group).
Don’t know the specific chapter but the Sanada storyline (their past) starts at chapter 18.

I’m not sure about that. Reiji cared for his half brother and his family back then, and it doesn’t seem to have changed since he wants revenge for his half brother’s murder.
In this last chapter, Reiji seemed shocked that Wang said he has Yuki as a pawn against Hyoudou. But he still went ahead with his plans for revenge disregarding Yuki’s safety.
I believe he would try and save him, but he might be so stuck on revenge that he wouldn’t flinch if something bad happens to Yuki...

I was rereading this because I’m a masochist, there’s a scene in a chapter (no idea which one but i know it was before we, the readers, found out about their family ties) where Wang berates and complains that Reiji for interrupting him from raping Yuki (it was a flash back and a follow up on when the scene cut off as he was about to kiss Yuki)
Reiji came in just after that saying there was an urgent matter.
While i think Reiji is very “ends justify the means” in his vengeance what the scene shows is he has a line. And is very sneaky.
I found it really interesting and looking forward to it baring fruit.

Oh he did, sorry for the confusion. This was before the second time Wang got him (after he killed the stepfather)
The latest chapter we see Reiji meeting Wang to discuss plans and Wang mentioned Yuki turning up (this is before he killed the stepfather)
I’d like to think Reiji added “protect Yuki without blowing cover” along with his revenge plans
Does reiji know he has a nephew? That it’s Yukio?