I'm on a Dcxpjo kick rn, so here's another idea
Tim is Jason and Thalia's cousin. Beryl Grace and Janet Drake were sisters. They had a falling out, so Tim doesn't know about Jason or Thalia (or if he does, he doesn't know they're related)
How do they meet, you may ask. Diana. She decides to introduce her half siblings to the league (it doesn't rly matter why).
This is an au where Jason grace either doesn't die, comes back to life, or ends up in valhalla bc I want him to be there too. Another thing i rly like is the idea that Jason looks a lot like Beryl.
So Jason and Thalia meet the league, and red robin (plus maybe some of the other bat kids) is there. rr looks at Jason grace and is like wtf why does he look like my mother? beryl and janet look similar from what little description we get of beryl (blue eyes blonde hair), and while I can't find a description of janet, she seems to often have blonde or light brown hair.
The bats, being as paranoid as they are, do a dna test and find out that Tim has cousins.
Feel free to add any ideas