Questions about the validity of the Cinderella effect aside, the OP knows the couple and can probably make a more informed judgement about this.

Of course, you can't tell this perfectly. But if the OP is anything more than casual acquaintances with the couple, I would say specific evidence probably overpowers the general case.

Has this been demonstrated in adoptive parents, though? Having only adopted children seems as though it might bias things in a different direction.