Complex sentence: Apart from his elder brother, John Mowbray had three sisters, Elizabeth, Margaret and Isabel, who, thanks to Thomas'treason, had to be found husband's in severely straitened circumstances.
Split into simpler sentences: Apart from his elder brother, John Mowbray had three sisters, Elizabeth, Margaret and Isabel. Because of Thomas' treason, the sisters had to be found husbands in severely straitened circumstances.