Blood harmonies: there’s magic in ‘em, literal and figurative — nature and nurture, love and rivalry, atmospheric alchemy born of living room dust and familial mishigas. Blood harmonies alone would be reason enough to cheer the surprise debut of Snocaps — Waxahatchee ’s Katie Crutchfield and twin sister Allison Crutchfield of Swearin’ and P.S. Eliot, the best-known of their teenaged pop-punk sister acts. Of course, Sister Katie is also coming off two of the decade’s best albums, St. Cloud and