What incorrect supposition did Jack make?

Jack was found inside a black handbag in the cloakrooms of Victorian Station, so, nobody knows about his real family, as a consequence of that, he thinks that he hasn't got parents. But, Lady Bracknell comes to Mr Worthing house, and, she asks to Miss Prism about the male baby that she lost. This is why, Miss Prism tells to all of them the real story and Jack realizes that his brother is Algernon, since that moment his friend, and his aunt Lady Bracknell. So, during the play, Jake makes that incorrect supposition.

Can it be? Someone has another answer?