She isn't sure

"I-I'm not sure." Sae-byeok told you she doesn't know whose baby she is carrying. You and your father both had unprotected sex with her and came inside her on the same day. Sae-byeok wasn't sure whose sperm had succeeded in fertilizing her eggs.

You put your hand on her belly, "I hope it's mine." You felt an oedipal desire to overcome your father. You wanted your sperm to defeat his and the child growing bigger in Sae-byeok's belly to be your son instead of your half brother.

But you won't know whose sperm won until the baby is born.