Batfam AU where Jason never dies, so Tim doesn't join the family the standard way. Instead, he continues pouring most of his time and energy into his photography, eventually becoming known as a popular photographer for events and all that. So now, picture this:
Tim gets hired to be a photographer for a Wayne gala. Obviously, he's ecstatic, because he can take pictures of Batman, Robin and Nightwing and be in their presence for a whole night. Since Tim is so naturally talented in stealth and taking pictures unnoticed, the second one of the fam realises this they're like: this kid is good.
Tim manages to go unnoticed by all 3 of them (all bat-trained, one literally batman) multiple times during the night, and even when he is noticed, he disappears before they can manage to get a good look at him; to the sheer amazement of Dick and Jason.
Jason, (very discreetly putting snacks in his suit pocket): i know you're under the table, kid.
Tim: don't mind me, Mr. Todd-Wayne, sir, just taking a few pictures
Jason: right... Jason's fine, and what pictures were you taking from under the table?!
Tim, showing him perfectly good shots of him: these.
Jason: how did you get that. it looks like you took it from the rafters
Jason, glancing at the ceiling: ...what?
Dick, hearing a very, very faint camera shutter from behind him:
Dick, turning around and finding no one there: what the actual...
Dick, getting the feeling of being watched and whirling around to find Tim staring at him from across the room: ... huh.
Jason, pulling Dick aside: you see that kid too, right?!
Dick, nodding: the camera kid, yeah?
Dick: he's one of the hired photographers, apparently. one of the best in his field, despite his age.
Jason: he's good. like, really good. snuck up on me 4 times already, the little bastard.
Dick: you too? i swear he's constantly watching. it's creepy how well he can sneak past both of us.
Jason: you don't think...
Dick: no. B would've told us.
Dick: did he get another kid and not tell us somehow
Bruce: what do you mean another kid?
Jason: you heard us. did you adopt another kid and not tell us?!
Bruce: no?? how would I even?? ... what's this about?
Dick: one of the photographers has managed to sneak up on both me and Jay multiple times already
Jason: he also can't be more than like. 15 or 16. so forgive us for assuming you took another one in.
Bruce: do you know his name?
Dick: in our defence, he's very hard to catch. i wouldn't be surprised if he's snuck up on you, too.
All of them, whipping their heads toward the sound only to find nothing but air:
Tim, smiling from the other side of the room:
Jason: do you see what we mean?!
Cue an entire night of shenanigans where it's just Dick, Jason and Bruce trying to catch Tim and learn about him. Upon finding out who he is and where he lives, Dick immediately asks to keep him as an honorary member of the family. Jason is hesitant at first but at some point Tim calls Bruce Batman instead of Mr. Wayne on accident and Jason laughs so hard he's basically won over. Bruce can do nothing but watch as Tim proceeds to come over almost every night for sleepovers and is coddled by both of his sons. And he can't deny, the kid's investigation and stealth skills are top tier. By the time Dick and Jason both start referring to Tim as 'their younger brother' Bruce has just accepted his fate.
