Will a brother be held accountable for not advising his brother about the obligation of prayer, knowing that their father is giving advice?

Advising and enjoining what is right and forbidding what is wrong is a collective obligation (fard kifayah). If a father fulfills this duty towards his son who does not pray, and the son does not respond, then there is no sin upon the other son, unless he has the ability to compel his brother to pray and fails to do so.