Otoha: It was recently shown that Nodoka knows that Otoha knows who the Girl Who Cannot Breathe (aka Sekai and Ami's mom) is:
Otoha however is apparently trying to hide this.
Then there's how Otoha seems to see Sensei as a little brother, while he
was Sekai's little brother in law. Both Sekai and Otoha also seem to be shotacons, and shitty when it comes to love. There's something strange going on here, but it's not clear what or why.
Niki and Noriko: They grew up with Sensei. They were next door neighbors and he was basically a part of their family long before he dated Niki, and since he was "with" Sekai at least:
When Sensei was around 18 and Niki 16, they dated for 5 years, then the accident happened, Sensei ghosted her, and after 8 years they reunited.
Some stuff on Noriko:
Overall: The Nakayamas are basically family. Niki is like a younger big sister, and Noriko a little sister. Niki is also his childhood best friend who he used to watch anime and go play in the woods with.
Sensei has tried to hide things from Niki mostly because he cares about her and doesn't want her to see how he really is. He doesn't feel like he deserves her.
Niki is basically everything that is good for him. Nodoka is the opposite. She makes him feel numb. Niki makes him want to be better. They are basically the Dark side (Nodoka) vs the Light side (Niki) imo. Nodoka wants him to accept his horribleness. Niki wants him to grow up.