The Prophet ﷺ asked Abu Bakr for ʿAisha's hand in marriage. Abu Bakr said "But I am your brother." The Prophet ﷺ said, "You are my brother in Allah's religion and His Book, but she (Aisha) is lawful for me to marry."  

أَنَّ النَّبِيَّ ﷺ خَطَبَ عَائِشَةَ إِلَى أَبِي بَكْرٍ فَقَالَ لَهُ أَبُو بَكْرٍ إِنَّمَا أَنَا أَخُوكَ فَقَالَ أَنْتَ أَخِي فِي دِينِ اللَّهِ وَكِتَابِهِ وَهْىَ لِي حَلاَلٌ