Mary Pew, who married Thomas McClain, was indeed the daughter of Thomas Pew and the sister of John Pew (1770), who married Elizabeth Vaughn. There were two other brothers, whose names I don't recall right now. After Thomas McClain died about 1812, she remarried to Asa Arnold. Mary and Asa are my ancestors. I have the names of all her children by both husbands. Mary was born 1778 in New Jersey, according to a later census record (as Mary Arnold). The Pews moved to Westmoreland Co, now Fayette Co, PA just after that. I believe she married Thomas McClain in Washington County about 1799, but this year is not certain. They certainly knew each other from Westmoreland Co. days, where the McClains also lived. Thomas and Mary probably moved to Mercer Co PA about 1800, the same time as John and Elizabeth Vaughn Pew. We know what happened to her other brothers, but again, I don't recall right now.
I have a ton of information on the Pews and some on the McClains, which I wil be happy to share with you. Please send me an e-mail at my AOL address and I will send the material to you.