Is it permissible for the husband of a wife's aunt to compel his wife's nephew to pray Fajr in congregation, knowing that he prays all other obligatory prayers in the mosque, except for Fajr sometimes due to fatigue?

Praise be to Allah Almighty who has prepared for you someone to assist you in His obedience and wake you for prayer. Performing prayer on time despite tiredness carries a greater reward, so thank your aunt's husband for his deed. You should take measures to ensure you wake up if you sleep before the prayer time begins. Your aunt's husband is not obligated to wake you as soon as the time begins, unless the prayer time is narrowing or you intentionally sleep after the time has entered. The waking should be done in the gentlest manner. Congregational prayer in the mosque has great merit, but your aunt's husband cannot compel you to attend it in the mosque, though it is preferable that you obey him.