Some comments on the song in kytrad's version:
I don't know the phraseology of the more bawdy traditions in English folk songs, but following German traditions it is clear that here not brother and sister are listening to bird songs.
The bow moving to and fro on the fiddle clearly hints to cohabitation; and to hear the birds sing [in heaven] is a metaphor for the excitement at the end of the act. Here the nightingale is chosen because of the dark hours, and since it is the undisputedly most wonderful singer.
So much for the nightingale's nocturnal song of love and lust.