Complex sentence: She was the niece of John Comyn, Earl of Buchan to whom she was also heiress, and after his death the Earldom of Buchan, was successfully claimed by Alice's husband Henry de Beaumont, Earl of Buchan, by right of his wife.
Split into simpler sentences: She was the niece of John Comyn, Earl of Buchan to whom she was also heiress. The Earldom of Buchan, however, was later claimed by Alice's husband Henry de Beaumont, Earl of Buchan by right of his wife.