I am a daughter of Prajapati (the lord of all creatures, Brahma) and my name is Devasena. My sister Daityasena has ere this been ravished by Kesin. We two sisters with our maids habitually used to come to these Manasa mountains for pleasures with the permission of Prajapati. And the great Asura Kesin used daily to pay his court to us. Daityasena, O conqueror of Paka, listened to him, but I did not. Daityasena was, therefore, taken away by him, but, O illustrious one, thou hast rescued me with thy might. And now, O lord of the celestials, I desire that thou shouldst select an invincible husband for me.
O hero with long arms, I am Avala (weak) but my husband must be powerful. And by the potency of my father's boon, he will be respected by gods and Asuras alike. That manly and famous and powerful being devoted to Brahma, who is able to conquer all the celestials, Asuras, Yakshas, Kinnaras, Uragas, Rakshasas, and the evil-minded Daityas and to subdue all the worlds with thee, shall be my husband.
The Performer of a thousand sacrifices then thought of Devasena, whom he has rescued before. And considering that this being (Skanda) was undoubtedly destined to be the husband of this lady by Brahma himself, he had her brought there, dressed her with the best apparel. And the vanquisher of Vala then said to Skanda, 'O foremost of gods, this lady was, even before thy birth, destined to be thy bride by that Self-existent Being [Brahma]. Therefore do thou duly accept her lotus-like beautiful right hand with invocation of the (marital) hymns.' Thus told, he duly married her.
It is also said that Siddhi and Biddhi are the daughters of Brahma as per Ganesha Purana.
Now,it is known that Sati,who is Daksha's daughter,is Brahma's grandaughter.Hence,any daughters of Brahma is Sati's aunties.This means that Devasena,Siddhi and Buddhi are Sati's aunties and hence,the grandmothers of Ganesha and Skanda.In that case,how can Ganesha and Skanda marry their own grandmothers?