Complex sentence: First was the problem of his nephew Orhan (1281-1362) the son of Osman I and Malhun Hatun and Mustafa Çelebi stood in the way of peace for Mehmed.
Split into simpler sentences: First was the problem of his nephew Orhan (1281-1362) the son of Osman I and Malhun Hatun. Orhan stood in the way of peace for Mehmed.