Complex sentence: Her mother was the sister of Fenella Trefusis, who was married to John Herbert Bowes-Lyon, the Queen Mother's brother and the second son of the Earl of Strathmore--making Idonea a niece of the Queen Mother, by marriage.
Split into simpler sentences: Her mother was the sister of Fenella Trefusis, who was married to John Herbert Bowes-Lyon, the Queen Mother brother and the second son of the Earl of Strathmore. Making Idonea a niece of the Queen Mother, by marriage.