So, DD3 didn't go to bed on time due to a late sleep. DD2 wasn't tired. DD1 had had her melatonin so went to bed.
DD2&3 were playing so lovingly, reciprocal, spontaneous play.
DD1 has never had that spontaneity and imagination and interaction and it hit me like a brick.
It is hard to watch something you should delight in and feel sadness.