Complex sentence: Within thirty weeks of marriage (March 1362) Mary became lethargic and died, she was buried in Abingdon Abbey with her sister Margaret, Countess of Pembroke, who died weeks before her.
Split into simpler sentences: Within thirty weeks of marriage (March 1362), Mary became lethargic and died. She was buried in Abingdon Abbey with her sister Margaret, Countess of Pembroke, who died weeks before her.