Sin is also a common Scottish form of "Sun".  John's glossary is good, but not so comprehensive as Warrack's Chambers Scots Dictionary and his Scots Dialect Dictionary, to which I refer in such matters.  Biek is not given, but would seem to be the same as Beik, which is defined under Beek, "to warm before the fire; to make warm; to bask in the sun or warmth of a fire; of the sun: to shine brightly; to add fuel to fire."  Also, "to bathe, foment".  Forenent: "opposite, facing, over against, in opposition to".  Hence, "To bake/bask in front of (beneath) the sun."  I think that this is the obvious interpretation of the line; no other had even occurred to me until Abby mentioned it.  Bruce's reading may be equally justified, but I do feel that mine makes better narrative sense given the context.  I don't have Child's notes at present, so I don't know if he expressed an opinion.

Malcolm