OH BOY so the Mabel AU is an AU I've had rotating in my head for months now and have discussed it with multiple friends in campfires. SO the premise is that Tim's stint as Robin is... different.
Tim is even more standoffish; he is adamant that he needs to be back home earlier on weekends, when he gets extremely injured he insists on going back home, only calling to check in, and only lets Bruce or Alfred visit during the weekdays. Bruce isn't sure how to deal with this, but tries to take the distance with ease, squashing his worry. Tim is a lot more independent and insistent that he can take care of himself. For plot reasons, no-one ever digs deeper into this.
When Janet Drake dies and Jack is in the hospital, Bruce is about to offer to foster Tim of course, but nope, Tim has already solved that. He tells him that it's okay, there's a trusted adult already looking after him, and he provides Bruce with all sorts of evidence. Again, Bruce and Alfred accept it suspiciously and are starting to look into the Drakes, but they find nothing out of the ordinary. Bruce isn't spurred into action until Tim is attacked by Jason at the tower.
A day or so later, someone knocks on the manor door. When Alfred goes to open it, he's greeted by a little girl who cant be older than 8, with big blue eyes and feathery black hair.
“My brother said that if I was ever in trouble, to go to the Waynes,” she says carefully, sweat on her forehead and dress smudged with dirt.
“And who is your brother?” Alfred asks.
He knows the answer though, as the little girl stares at him with too intelligent eyes.
“Tim Drake,” she replies.
And everything clicks into place.
TLDR; Tim is an older brother to a younger sister called Mabel Jane Drake who he adores and tries to keep out of danger no matter what, but she loves her big brother just as much, and ends up meddling to keep him safe. I just think the Batfamily needs more little sisters/daughters to dote on!!!
I've actually started writing it for a friend's birthday, and hope to release it soon!