Could be the door of the bathroom, would be a waste not doing a shower scene as continuation, all the darkness imply it's late so could be just after the titjob, at this point Luke is sleeping, David is probably sleeping too and even if he were awake Alice would cockblock him, Peter is... I don't know, they don't even mention him after the whole Silvia thing I think, so it's Aaron spying on Sarah while she takes that shower, could be also Uncle Peter just getting home and spying on Sarah and Aaron while they take a shower, even if Peter has his own agenda he clearly wants to see Aaron fuck Sarah for some twisted reason.