Erm, I guess that answers any questions if Saeko's got part-time jobs outside of supermarket work. Synopsis says she's uneducated, but she's sexy and as we see, open to sex even with ugly men, so her being some kind of sex worker on the side wouldn't be a stretch. We didn't see her get paid for now, but she could just be setting up this man for some later dirty work (instead of payment) against her enemies instead.
Given that Saeko looks pretty crazy, there are two possibilities regarding Yui: either she's her real daughter (maybe from a customer), or she's crazy enough to have kidnapped someone else's child to raise as her own, because Yui really looks nothing like her.
Like, imagine if she was the yandere homewrecker affair partner in a love triangle featuring a married couple, where she ends up murdering the wife soon after she gives birth (the husband dies too) and she took their newborn to raise as her own child.
