There is a literary rule when creating a story called Chekhov's gun which states that every element in a story should be necessary and contribute to the overall narrative. The quote is "If in the first act you have hung a pistol on the wall, then in the following one it should be fired. Otherwise don't put it there.".
Are you actually comparing Gab to Chekhov's gun? First of all, Gab did, and still potentially does have story value. Even if he didn't, you don't just discard or kill off characters once their original story purpose has concluded. We're talking about a character in the story, not some object or tool. One who is very close personally with the MC.
And you are vastly overstating the importance of Chekov's gun as a literary rule. Take The Lord of the Rings for example. The only hobbits who have a "use" in the story are Bilbo, Frodo, Sam, Merry and Pippin. Yet Tolkien introduces an entire hobbit town with lots of families. Why do they exist? Not for some use in the plot. It's called world and character building.
Even if Gab had no use plot wise, he's still an important character to the MC, being his brother and only living family.
As far as I can remember, we have no idea what Noel was doing before the MC. All we know for sure is that she was with her sister and Gabe. Do we know what the relationship between Noel and Gabe was? As far as I am aware we don't. As for being NTR, no, it is not NTR even if Gabe and Noel had a relationship before the she and the MC met. However, it also isn't NTR for Nael and the MC to have a relationship if something were to happen to Gabe.
As for Noel being bound to Gabe, it is a possibility. We know that powerful dark ones like the primodials can damage the bond when attacked (Nael says so during the fight between Gabe and Matt). It is completely possible that Arsa severely damaged her bond with Gabe when she attacked in her true form and then she bonded with he MC because he felt like Gabe. As you can see in the picture I posted down below, the first interaction Noel had with the MC when she though he was Gabe isn't exactly the most platonic interacion. I mean, when is the last time a girl you are just friends with came up to you and tenderly put her hand on your cheek and looked in your eyes. My guess is never, or maybe my friends just arent that affectionate lol.
Speculating without evidence that Noel and Gab had a sexual relationship is just that, pure speculation. She's a dark one going feral, she instinctively was bonding with the first person she could. As far as it not being a platonic interaction she was forming an intimate bond with MC, of course that's not platonic.
And frankly it doesn't matter one way or the other if they had a sexual relationship in the past or not. Noel is currently with the MC and even if they did have an intimate past, Gab has shown no sign whatsoever of trying to get her back. He's been nothing but supportive of his brother the entire game.
And as far as powerful dark ones damaging the bond, damage is not the same as outright severing the bond. Plus Noel, while weaker than Arsael, is still a primordial herself and hardly defenseless. Even when attacked by Tessah and Matt at their full power it didn't sever the bond Noel had with MC. If Tessah who is far more powerful than Arsael didn't have enough juice to forcibly sever the bond without killing the human or dark one involved, Arsael damn well couldn't have. Their bond was only severed when Matt fully erased Noel by killing her.
