Is big head rourou's brother? Is it yiyun's child?

Yess. Since Jinyao switched his and yiyun's sample, that baby probably yiyun's

Yes, (spoiler warning to others) its Yiyun's genetic child. His sperm that was stolen after Feng exchanged it for his own for Rourou was mistaken for Feng's and his aunt used it to make big head so she could lead the family with him.
So yes to both, Big head is Yiyun's son AND Rourou's brother

Oh okay thanks