Complex sentence: Charles first met Lady Diana Spencer in 1977 while he was visiting her home, Althorp, as the companion of her elder sister, Sarah; he did not consider her romantically until mid- 1980.
Split into simpler sentences: Charles first met Lady Diana Spencer in 1977 while he was visiting her home, Althorp. He was the companion of her elder sister, Sarah, and not consider Diana romantically until mid- 1980.