Complex sentence: An exception is made, however, if the female who breaks the male line of descent is a heraldic heiress--a woman who has no brothers, or whose brothers have died without issue.
Split into simpler sentences: An exception is made, however, if the female who breaks the male line of descent is a heraldic heiress. A heraldic heiress is a woman who has no brothers, or whose brothers have died without issue.