Complex sentence: He was the eldest son of a judge, Alexander Boswell, Lord Auchinleck and his wife Euphemia Erskine; he inherited his father's estate Auchinleck in Ayrshire.
Split into simpler sentences: He was the eldest son of a judge, Alexander Boswell, Lord Auchinleck and his wife Euphemia Erskine. AS the eldest son, he was heir to his father's estate of Auchinleck in Ayrshire.