Does delaying the wife's making up of the days of Ramadan fasting missed due to menstruation until late Shaaban, out of forgetfulness, constitute a sin?

Your wife must make up for any missed fasts from the last Ramadan (definition) if she did not fast them before the current Ramadan began. Along with making up the fasts that were delayed from the current Ramadan, she must feed a poor person for each day (a mudd of wheat) or half a sa' of other food. This feeding is an expiation for the delay if it was without a valid excuse. However, if the delay was due to a valid excuse, then only making up the fasts is required. If her forgetting to make up the fasts resulted from negligence and dereliction, it is not considered an excuse. But if it was not due to negligence, we hope she is excused, and there is no sin or feeding required of her, based on the Hadith (definition): "My Ummah is forgiven for error and forgetfulness." If she already made up what was due in the last days of Sha'ban, then nothing is required of her.