It would be nice to give her none of that explanation, you need to have room temperature iq if you don’t realize there is a possibility their siblings go to the same childcare after the classroom talk
while i don't like her nosiness, i do think that her behaviour isn't as egregiously unreasonable as it initially seems, since to be fair:
- when she asked FMC earlier if she has any siblings, she just made an X with her fingers. it's not unreasonable to take that as a "no".
- she could have noticed FMC whispering to MC during class, making her suspicious of a potential "secret relationship" between the two of them (partially correct).
- the combination of the above two points, and of them talking in front of the daycare (plus a little bit of confirmation bias) may have led her to the conclusion: "secret couple leave school separately as to not arouse suspicion, then gf waits for bf in front of his siblings' daycare so they can walk home together." - all whilst her subconscious conveniently ignores conflicting information such as MC only having 2 siblings, or the third kid looking awfully similar to FMC.
and concerning why she was even there in the first place - I'm willing to give her the benefit of the doubt and assume that it's something along the lines of "her younger sister also happens to go to the same daycare, and she's picking her up today due to a one off request from her mom (who usually picks her up, but couldn't make it today)".
but honestly I wouldn't be surprised if she really did just stalk FMC for no real reason other than "she wanted to know what she does after school".