Addressing the question of Michael's biological relationship to Sonny, it was explained to me that Michael is, in fact, Sonny's biological son. However, the decision to conceal this fact stemmed from Sonny's desire to keep certain things a secret. First of all, Michael's mother was quite young when he was born, around 15 years old, while Sonny was 34. Furthermore, Sonny was married to another woman and having a child out of wedlock was not taken lightly either.

But you had to play it cool, had to do it your way
Had to be a fool, had to throw it all away