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I HOPE THIS CLARIFIES WHAT IS HAPPENING.
Feng had a semen sample and Yiyun had a semen sample. Feng switched their semen samples around. Rourou is Feng's biological son. Yiyun raised Rourou believeing Rourou was made with his semen sample, until Feng told him otherwise. They are now dating and raising Rourou together
Yiyun's semen sample was stolen. The thief thought they stole Feng's semen sample.
The grey haired aunt is blood related to the Feng family. She can't have children. She needs a heir that is blood related to the Feng family. She stole Yiyun's semen sample thinking it was Feng's and put it in a surrogate mother, that lady just gave birth to the baby named Bighead. The aunt thinks she now has a child that carries the Feng bloodline. But the baby is actually related to Yiyun and not Feng. Yiyun doesn't know that his semen sample is also a baby now.
Rourou is the biological son of Feng. Bighead is the biological son of Yiyun.
I hope that helps.