Alice McDermott's powerful new novel wittily captures the social, political and spiritual upheavals of the mid-twentieth century through the story of a family, and the changing world in which they live.      While Michael and Annie Keane taste the alternately intoxicating and bitter first fruits of the sexual revolution, their older, more tentative brother lags behind, until he finds himself on the way to Vietnam. Meanwhile, Clare, the youngest child of their aging parents, seeks to maintain an almost saintly innocence.        After This, alive with the passions and tragedies of a determining era in our history, portrays the clash of traditional, faith-bound life and modern freedom, while also capturing, with McDermott's inimitable understanding and grace, the joy, sorrow, anger, and love that underpin, and undermine, what it is to be a family