Keep in mind that Sensei's sister in law, was also like a mother figure, a girlfriend, and an onahole to him.
It's not really that Touka is suddenly becoming a mother figure, it is that Sensei seems to be forcing Touka into a matriarchal role for him, which Tsubasa seems to think would disappoint Touka:
Touka is more or less just being groomed into this role by those around her:
Especially since it's what Sensei wants from her:
Yasu is molding Touka into being a guardian, because she's too wholesome to let her freeze to death and drink alley leaves.
Tsubasa is molding Touka into being a landlord/landlady, because it's expected of her to eventually carry the family name.
Sensei is molding Touka into a motherly role because this is what he actually wants from her.
She's basically becoming Mommy Touka because she's too gullible and wholesome to resist, and well, she's good at it.
Ami and Niki are in similar situations when it comes to Sensei. I guess so is Otoha, and Nodoka in a way. Although, he has more of a big sister relationship with Niki and Otoha, even though they are both younger.