Spoilers:
No they’re brothers not father and son. When Zhenya was talking about his backstory and his younger sister and younger brother Sergei was the younger brother that went missing that they thought died. He was kidnapped and became an experiment.
The queerbait they’re talking about is the scenes in the most recent dungeon where the npc’s would assume and talk about the two being a gay couple and Gru being their daughter as well as the random blushing from Sergei.
Though I understand the people that think they should be together it becomes disgusting to those who read the novel or got spoilers that Sergei is a minor and the fact that the two are brothers. The issue is that the manhwa hasn’t gone over those subjects just yet which is creating uncomfortable misunderstandings.

The queerbait with Zhenya and Sergei