They don’t stop being siblings just because they happen to rule a country. Poke!!
 
I personally imagine this devolved into a tickle battle/pillow fight/verbal jousting match very shortly after this. It ended in Ponyville, where an utterly baffled Twilight walked into her library only to see it in utter disarray; split into two sides by impressive book forts, strewn with more pillows than she owns, an inexplicable catapult and these two in an exhausted cuddle pile on the floor. Twilight promptly quietly closes the door and goes back to bed.