https://steamcommunity.com/dmca/create

But be warned that it might tie up both games in bureaucracy.
You might hope that steam has the timestamps readily available to prove the sequence of events - though I'm a bit skeptical of cloud storage about that stuff. So you might want your own evidence from your own storage.

It'd be good to try to find someone who has gone through it on steam, and of course a lawyer, if you can afford it.