Just replace "big sis" with "mommy" and she most definitely does, just because she uses another term for it doesn't change the possessive, infantilizing, incest like undertones of her behavior.
It's not Null that should be worried, it's everyone around him. Just because she can't fuck with his head directly doesn't mean she can't fuck with the heads of everyone around him. It's like when your crazy gf starts lying to your friends to use social pressure to manipulate you, only Jean can directly fuck with their minds.