It's hard to say Love.

March 27 for a due date corresponds to an LMP of June 17. So your brother would have had sex BEFORE her last period. In that case he is NOT the father.

Due dates are based on a 40 week gestation. It doesn't matter if the baby is born before or after that - it's just an approximation.

Do you know when her LMP was? If it STARTED on June 17 he is in the clear.