Oh. My. Celestia. What.

Those lyrics.

In fact, I just thought I might
pop the question there that night.
I was kissin' her so tenderly,
but woe is me!

Who woulda guessed
her family crest
I'd suddenly spy
tattooed on her thigh.

And son of a gun,
it's just like the one on me! Tell me...

How was I supposed to know we were both related?
Believe me if I knew she was my cousin we never would have dated.

Let's analyze this.

...family crest...
...tattooed on her thigh...
...just like the one on me...
...she was my cousin...

Cutie mark. Three apples. Three balloons. Possibly cousins.