Just an observation of my own - not from a learned book but my sister and I, only 18th months between us, bickered and fought throughout our childhood, except when our mutual friend and near neighbour, younger than my sister, older than me joined us to play. Then all three of us got on well together. Same thing happened when my 2 DDs were young. They were always fighting over toys, who got the top bunk etc. until their mutual friend came to play. Again, all was sweetness and light between the 3. They would play in the bedroom or the garden for hours without a single upset.
Now I have a big brother, wee sister pair of Grandbairns who squabble constantly and keep complaining 'It's not fair' until cousin K (my other GD) appears and peace reigns. It is far less stressful and tiring to have all three staying together than cope with the 2 siblings who argue constantly. Perhaps your GSs regard their Father as the 3rd playmate? Do you have any friends with 4yrold who could come to play regularly?
PS My DD's former childminder also said that her own two children fought like cat and dog when she stopped childminding, whereas they had played happily for years when there were other children in their home.