St. Vincent - and check out her album with David Byrne - or does Byrne's masculinity negate her contribution? The Kopecky Family Band (two lead singers one male one female) I see a lot of blues artists have shown up in this thread but we've neglected modern ones: what about Regina Spektor (no relation to Phil)?
like even Britney Spears sings songs that aren't about love/relationships, see her song "Lucky" for instance or (if it counts) she covered the Rolling Stone's "(Can't Get No) Satisfaction" and that's not about love; lust at most, and I think the song can be interpreted on a societal, not interpersonal, basis
the kills fuck even the spice girls : "wannabe" is as much about female friendships as it is about potential relationships; they sang a song about mothers for goodness sake what about 4 non blondes whose hit song is political what about the fucking cranberries