Widobrave Rights

Firstly, the mother/son dynamic between Caleb and Veth is a thing fandom has blown entirely out of proportion. In canon, Nott only mentions it once in a context of "Caleb's not my parent, if anything I'm his because I take care of him" but that was less claiming Caleb as her child and more trying to explain their dynamic to the rest of the Nein. She wanted to make them understand that Nott's not a child and of the two of them Caleb was the more vulnerable. Neither of them ever bring up a parent/child relationship between them after Nott clarifies that she's the protector. Caleb never indicates he thinks of Nott as a mother and only once brings up thinking of Nott as sort of a younger sister.

These conversations also take place early in the campaign when the characters are still settling into their roles. Using only that situation to define Caleb and Nott's relationship completely ignores their development in later parts of the campaign where this idea is contradicted by other emotions and feelings that are brought up. These changes and development make Caleb and Veth's relationship complicated to say the least but parent/child is not the end vibe, if it was even the starting vibe in the first place.

Veth canonly finds Caleb attractive. She states as much several times and admits to Beau she thought him attactive from their first meeting. She also canonly kissed him and calls Caleb "the second love of her life" (both happening after Veth's reunited with her husband no less). Caleb also has an emotionally charged conversation with Beau that implies even if he doesn't feel the exact same way Veth does, that his emotions towards her aren't completely platonic. Which is why Caleb's so weird around Yeza and backs off when he's there. Caleb knows the things he and Nott do together and say to each other aren't things appropriate to do and say around her husband.

Yeza has every right to be uncomfortable around Caleb in return because Caleb isn't their or even just Veth's adopted son in any way, shape or form. Caleb can't even just be Veth's friend to Yeza because as all three of them are uncomfortably aware, Caleb and Veth pretty much conducted what amounts to an emotional affair, intentionally or not. To make matters worse, it continued past Veth and Yeza reuniting and Yeza picked up on it. Hell the rest of the Nein picked up on it. 

That Yeza doesn't really stop it says a lot about him as a character both from the idea that he knows how vital their relationship is and doesn't stop them from that comfort (he's a really good guy) and from a perspective that maybe, just maybe, he's not 100% sure who she'd pick if asked (Yeza lacks confidence in a similar way Veth herself does).

This is further complicated as Veth canonly doesn't even know who she'd picked if asked, having flip-flopped on the subject herself everytime it came up (once saying she wouldn't be separated from Caleb only to turn around and say their time together had to end because she had to go home to her family).

Yeza and Caleb's discomfort comes from this place of "this man is me in her other life" and "he's as vital to her as I am" and "I'm not used sharing that importance". It comes from a tenuous balance where both men know "if I rock the boat, one of us is drowning and I don't want that for either of us but more importantly, I don't want that for her".