I can tell you it seems to be a phase and you are right to assume it is to do with attention and the other child, she is feeling left out, you may think you give her as much attention but she really needs some one to one attention,perhaps the older one is asked to share her precious toys with the younger child this does happen a lot, and can cause resentment , sharing is good but not all the time ,its okay for her to say no if she wants her own toy .Let Dad have the younger child whilst you play some games and have fun with the older one. She may have got into a habit of moaning/complaining so the best way is to ignore that.. she does it for attention and I bet you give her it, dont reward negative behavior, focus on her positive behavior and praise her when you see her doing something right. Let her bake with you, help you do stuff around the house, but dont feed into the sulking, say nothing let it go. .