Despite everything I have read about squirrel behavior, Pookie has allowed her adult son (16 months old, sexually mature) to join her in her nest for winter.  She lost her late summer "fall" litter and would have spent winter alone.  She originally followed him to his leaf drey and despite some squeals of protest on his part insisted on moving in.  A few minutes later I watched as both gathered extra leaves and "refurbished" the nest.  (I could just imagine the mother's conversation with her adult son upon visiting his bachelor pad " is this all the furniture you have, do you ever clean this place up?" etc.) The next night they both moved into her tree cavity nest, a far better site for winter. That's Pookie on the right.  I have heard that males will share nest for warmth, but never that females will allow a male to join them.¹  I have observed them grooming each other and  playing/wrestling, have yet to get that on camera.  Will be very interesting come mating season to see how she behaves toward him. 

¹  Females commonly nest together, usually in mother/daughter combinations.  In this way, it is thought that daughters often "inherit" prime nesting sites and surrounding territories.  Sons, however, are usually driven away as they approach sexual maturity and are know to  nest together with unrelated males during the dead of winter for warmth as Jen describes.