Casual fans may wince at Kate McGarrigle’s allowing her ex, Loudon Wainwright, to duet with his kids Rufus and Martha on “Schooldays,” the first cut on McGarrigle’s latest album. In proving himself to be a sporadically stellar songwriter, Loudon’s also revealed that he’s sketchy as a family man, and Rufus and Martha are Kate’s kids, too. But folkies are prone to forgiveness, and family has always been central to Kate and Anna McGarrigle’s music. In keeping with their lifestyle, the sisters’ concept of family on this “family” album is looseno, Linda Ronstadt and Joe Boyd are not bloodbut the prevailing vibe is universal. The way Kate, Anna, and Martha selflessly back Rufus on “Goodnight Sweetheart” is fairly typical of the way moms and sisters support the young bucks in their midst, and if family friend Chaim Tannenbaum wasn’t made to feel like one of the gang, he may never have told anyone that he wrote songs (“Time on My Hands”). The 21 songs are a mixed bag of traditional spirituals (“Dig My Grave”), pop standards (Irving Berlin’s “What’ll I Do,” sung by Rufus, Martha, Kate, and Loudon, who, it’s pointed out, have never even had Christmas together), and tunes penned by the participants. But mostly, this is a singer’s album. Emmylou Harris joins the sisters on Jesse Winchester’s “Skip Rope Song,” an instant folk-diva classic. And even if “Heartburn” isn’t as fabulous as Rufus thinks it is, I bet he delivers something timeless one day. He’s got good genes.