I'm not as experienced as other people, so I can't really add anything. Except she probably missed cuddling with her sister, and I wonder if there's teaddyu bears that you can put a hot water bottle in? That way it would feel more life like. My degu's alone now too...and since the other one died she always naps by the heater, or lately even ON me.
Let her run around the room and stuff, and eventually she may come to you for attention..bribe her with a treat! Once they've seen everything in the room, they get too bored not to see you! But I guess it's true all degus are different.
Good luck! And I'm really sory for your lost!