This chapter portrayed her as a former social outcast herself. On top of which, she referred to him as nephew in the flashback. It's not possible for her to be his mom.
@MasterKurosawaKakeru she became a neet cause she got pregnant with a guy in high school and attempted suicide nobody has mentioned what happened to the baby.
So ether it got aborted or to cope with what happened totally erased her memory regarding the baby after the baby was born and her sister acted as the mom to her baby instead.