All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Since that man's sons are Mahram (permanently unmarriageable) to this woman, who was their father's wife, it is permissible that they see of her body what is permissible for other Mahram men to see. Allaah Almighty says (what means): {And do not marry those [women] whom your fathers married…} [Quran 4: 22] Marriage is prohibited in this case only because these men are Mahram to the woman.
Allaah Knows best.