They are, in fact, twins. And they're lesbians.
In their first two albums (Under Feet Like Ours and This Business of Art) they wrote some pretty sexy songs about girls.
Tegan and Sara have been popular for quite a long time. But before they got that single on If It Was You, they were pretty much considered dyke rock.
I saw a So Jealous concert on InDemand recently and I could barely recognize them - they've toned down their look a lot. They both used to have a really stereotypical dyke look.