SISTER WATER by Nancy Willard ( Knopf : $21 ; 272 pp. ). All you need to appreciate this lilting, wacky, wistful novel is a distrust of developers, a belief in a hereafter that sometimes appears to residents of the here and now, and faith in angels--even the ones in charge of shuttling us from this world to the next. Ellen and Martha are Jessie's daughters, standing by as their elderly mother fades in and out of reality, wondering whether to sell their late father's business and museum to Harvey, who would like to develop both a shopping mall and a relationship with the recently-widowed Ellen. When they hire Sam, a man of few possessions but endless ideas, to take care of Jessie, he too pursues Ellen, who is still trying to cope with her husband's premature demise and her mother's imminent one. Willard possesses a delightfully wry voice--but unlike so many modern writers, she also has an ear for whimsy. In fact, fiction lovers who don't quite believe in wings might take a chance on this novel anyhow: Only dedicated stone hearts will get to the end without melting, just a little bit.