Nests, eggs, and young birds are protected by the federal Migratory Bird Treaty Act. So once the eggs are laid you need to leave them alone. But if she's already fledged young, then go ahead and remove the old nest and discard of it in the trash and place outside. (Wash your hands in hot, soapy water after handling the nest, they can contain mites, etc.). Putting up the streamers may be enough to scare her away from the window sill. But if you notice material starting to gather, remove it right away before she has a chance to build the nest. She should get the hint and try to rebuild elsewhere. Because she has been so successful at that location in the past, she might be fairly persistent in trying to build there. So you just have to be more persistent. Good luck.