Complex sentence: His grandfather, Middleweight boxer Mike Gibbons is regarded as one of the best boxers of all time, while his grandfather's brother Tommy Gibbons is a renowned Heavyweight boxer of his time.
Split into simpler sentences: His grandfather, Middleweight boxer Mike Gibbons is regarded as one of the best boxers of all time. While his grandfather's brother Tommy Gibbons is a renowned Heavyweight boxer of his time.