Found funny glitch. The Hidden Sister, If you Hired your mother Jennifer she can ask you look at daughter resume & its not lily now i've got A Hidden sister working for me same last name & all..
Edit, no secret sister is Recruit worker glitch ,<- look daughter resume event. thier no real secret sister in coding
the daughter recruit event simply checks the mother's age and child count. since your mom has 2 children (you and lily), and only one is listed on her relationships section (lily), it will count that as an open space for the game to generate a new daughter for that event.
if the woman is old enough (older than 34, looking at the crisis doc) and you've gotten them pregnant/they've given birth, they can have the daughter recruit event for
your own child (because she now has an open child slot), who will now magically be 18--because them giving birth doesn't automatically create a new 0-age daughter off in offcreenland who couldnt meet the criteria to be recruited, it just adds to their children count, which age allowing will open them to have the daughter recruit event.
a funny side effect that's been around for a while is that everyone leave their child with their mother so they can return to adulting, even if you know or regularly interact with said mother (such as your mother and aunt if you got your sister or cousin pregnant, the student you can tutor and her mom, etc).
these newly generated daughters will show the appropriate mother/sister relationship tags, but not aunt/niece (such as if your mother has a new daughter generated, your/her aunt and cousin will be labelled as acquaintences rather than aunt/cousin etc, recruited daughters also dont flag you as their father, since you dont appear to be considered a character as far as relationship tab is concerned)
now, being able to hire/possibly enter a relationship with your own (now magically aged) daughters might be considered a perk to some players who really want to lean into the incest angle, and might even be considerable as an option in the game (such as a tier 3 serum spec that ages any children the mother has to enable this without any headscratching, i guess?), but currently appears to be an odd knock-on effect.