You're right. He didn't know. So I doubt he would have cared one way या the other if Katniss's sister hadn't died. He would've just walked away without a सेकंड thought.
What happened to Annie? And Finnick Jr.? And what happened to Gale's प्यार for Katniss? या Haymitch? या whether there's any other islands/continents/ civilizations besides Panem?