There actually is something in the first half that if you read a little between the lines does seem like it might indicate some negligence by the other girl's mother:

>> Kaye thought it was cute and snapped a photo with her phone and texted it to Eme’s mom, who’s a friend of hers. She replied that if at some point the girls wanted to ride Eme’s sister’s new electric bike (it was a bat mitzvah gift), they were welcome to walk over and use it.

It was a new electric bike, and it was a gift for the sister, and the sister was probably 12 or 13 since that is the age for a bat mitzvah. I don't have any siblings, but from what I saw of friends who did growing up, a 12 or 13 year old who gets a gift like this is going to monopolize it for a while.

I'd expect then that Eme had little to no experience riding it, which Eme's mother should have known. At the very least that means they should not have been allowed to ride together, since riding with a passenger is much harder than riding solo.

Eme's mother than should not have invited the girls over to ride it, unless she was going to supervise to make sure they didn't do anything too risky for their skill level. If she did not supervise that might be negligent. If she did supervise but allowed them ride tandem that might be negligent.