Complex sentence: In the Boston area he built St. Catherine's Church in Somerville, Massachusetts regarded as a masterpiece, and St. Aidan's in Brookline, Massachusetts where he was a parishioner along with the Kennedy family and other prominent Irish-Americans.
Split into simpler sentences: In the Boston area he built St. Catherine's Church in Somerville, Massachusetts regarded as a masterpiece. He also built St. Aidan's in Brookline, Massachusetts where he was a parishioner along with the Kennedy family and other prominent Irish-Americans.