Well, it's all speculation at best. Child helps little except to slightly upgrade the lesbian theory a bit. He cites the local tradition about the song (again, the only chance of making sense to to refer back to this as much as the text.)

He cites a letter of June, 1781 by Major Barry, the proprietor of Lednock... "When I came first to Lednock," says Major Barry, "I was shewn in a part of my ground (called the Dranoch-haugh) an heap of stones almost covered briers, thorns and fern, which they assured me was the burial place of Bessie Bell and Mary Gray. The tradition of the country relating to these ladys is, that Mary Gray's father was laird of Lednock and Bessie Bell's of Kinvaid, a place in this neighborhood: that they were both very handsome, and an intimate freindship subsisted between them; that while Miss Bell was on a visit to Miss Grey, the plague broke out in the year 1666; in order to avoid which they built themselves a bower about three quarters of a mile west from Lednock House, in a very retired and romantic place called Burn-Braes, on the side of Beauchieburn. Here they lived for some time; but the plague raging with great fury, they caught the infection, it is said, from a young gentleman who was in love with them both. He used to bring them their provision. They died in this bower, and were buried in the Dranoch-haugh, at the foot of a brae of the same name and near to the bank of the river Almond" (He note the Major's date is certainly 20 years late.

OK. Two probs still with this. I've never come across "sin" as 'syne' or as 'since' used as a verb. "Syne" is used very liberally with restect to time functions but I can't fit 'syne' into this sentence in any comfortable grammer.

Further, to bask in the sun troubles me, too, as they clearly have been burried, both in the song and the oral tradition. "Fornent" is an odd word - I glossed as you found, it's most common usage "in the face of, in opposition to" but I think this can extend to "in spite of," & thus even "on account of." "Bake" in this context clearly implies hell. (Both of these only work if "sin" means 'sin' in the first place.