In 1x01, it seems like Richie was set on embarrassing Carmy in front of Sydney. He makes fun of Carmy for making a Panettone and when she doesnt fall for the ridicule, and when Carmy bites back about Mikey leaving the restaurant to him, Richie goes on this mission to make fun of Carmy in front of this new girl.
He's also the first to call out their closeness by calling Carmy her best friend in 1x02
He had to have noticed Carmy being a deer in the headlights when Sydney came back in 1x08 because he's the one that breaks the tension between them.
This is enough said. He's aware that mom and dad are fighting, and he calls it out. This is before he knew Claire and Carmy were hanging out.
Maybe the idea of Syd and Carmy liking each other died down when he saw the distance between them in 2x03-2x05.
Claire is with Carmy in 2x05, It's interesting to Richie only because he knows Claire and Tiffany are close. It's a way to get intel on Frank and Nat notices Richie's underlining motives.
When they start dating, Richie is almost proud to see it, because Carmy is doing something Mikey would have wanted (sidebar did Mikey want carmy and claire exactly, or did he just want Carmy to enjoy life outside the kitchen?)
Either way, Richie tends to catch on to things quickly, except for the thing between Sydney and Carmy. He's blinded by grief and can't see Carmy as a person with his own wants and needs. He could only see him as Mikey's little brother.
What's missing between Richie and Carmy is a heart-to-heart. In 2x09, after Richie says, "Yeah, I like Claire, Cousin," Carmy takes a moment. I also see the look on Richie's face that's very serious. Carmy wants to say something to Richie about his dwindling feelings for Claire (and maybe talk out his growing feelings for Sydney) but he holds back because Richie and Carmy aren't as close as Mikey and Richie.
It's heartbreaking that Carmy says Richie's name in 2x10- he wants to say something about not being really liking Claire as much as everyone wants him too, but Richie jumps at his throat.
When Richie and Carmy have a heart-to-heart-, it's going to hit Richie that Carmy has feelings for Sydney. His dislike for them early in season 1 and season 2- his grievances for Sydney closed his eyes to the truth. That is until 2x08, when he acknowledges that Sydney is a part of the family.
I know this meta was mentioned before, but I see more of Richie witnessing Sydney and Carmys' dynamic.
Willow is that kind of show where you see a glimpse of a scene in the promo where the characters are covered in dust and you think "wow this must happen after the big epic fight at the end where they all get dirty while battling" and then you get to the episode and its just right at the beginning with Boorman saying "camouflage" as a plan to enter the Immemorial City.
Why are they sharing casual banter?? Why do they look like the protagonists of an action movie walking away from an explosion? Why do they look like bros? 😂