At first impression, the song does sort of seem to be about incest, maybe between cousins. But, although the band may be aware that it'll give that impression, I think that it's actually a straight-up love song: he wants to marry her. By saying "you are my family tree", I think it's a very forward statement of confidence in their being so perfect for each other that, to him, it's like they're already family.
One of the possible interpretations of the lyric "my heart is breakin' me down" is that his heart is breaking down any resistance he has to that kind of commitment.
At first impression, the song does sort of seem to be about incest, maybe between cousins. But, although the band may be aware that it'll give that impression, I think that it's actually a straight-up love song: he wants to marry her. By saying "you are my family tree", I think it's a very forward statement of confidence in their being so perfect for each other that, to him, it's like they're already family.
One of the possible interpretations of the lyric "my heart is breakin' me down" is that his heart is breaking down any resistance he has to that kind of commitment.