IMO, it isn't that odd for Nathan to be the only child of three boys to have stayed with the grandparents on Sunday night. Depending on the grandparents personalities and energy level, having all the kids spend the night together may be a bit overwhelming or tiring. Additionally, I remember as a child that having your siblings spend the night with you seemed to take some of the fun out! Getting to be 'special' and stay the night there without your siblings meant you got all the attention for yourself. Perhaps the other boys simply don't have that same connection with the grandparents-- truthfully, different kids connect differently to their grandparents (at least, I see this in kids within my own family). Lastly, I wouldn't be surprised if the older brother was busy with his own plans such as summer camp early the next morning, a sleepover planned at a friend's house, etc.
I would love a more detailed explanation about why the brothers weren't there, but then again I really don't find it odd.