Little work kitty lost his sister. He’s very clearly depressed. I don’t know what to do.

I have a kitty at work that’s a little to feral to home. I’ve been working on getting him and his sister more comfortable with people so I can reunite them with their mother who I adopted. Fucking tragically, nobody has seen his sister for a week. I can tell he hasn’t either cause he’s been meowing constantly and refusing to eat. I just don’t know what to do. He’s been missing for the past two days but he does this often, he’s an explorer. I’ve gotten them both neutered and spayed and vaccinated so I’m not worried about them wondering off to breed. I’m just heartbroken cause I feel like there was more I could do. I don’t know. I’m thinking the worst. It’s also likely she wondered into someone’s house and they kept her, she liked to go inside places. But either way he misses his sister. Anyone who’s cats lost a sibling, do you have any advice? :(