You absolutely hate your sister and want to get back at her. An idea crosses your mind, why not grant her wish... Except "she" is not the one getting to experience it, instead force her to watch on with envy as someone else does...
But whom? You look back through the keyhole and stare at Amelia. You recall the times you fantasized about her being your bride, "till death do us part"? You realize that only yourself would do.
You run out of your home, locking the door behind you. You then throw your keys into a bush, easy to recover assuming someone knew they were there.
You then activate the device and swap bodies with your sister who was still inside with Amelia.
Now you have trapped your sister outside in your body. You take a few steps closer to Amelia, getting ready for the next step.
You feel weird as your new breasts sway gently from side to side, the void between your legs and the swing of your hips. You realize that you have one chance at doing this right and you aren't even OK with being a girl. If only you could do a trial run.
You remember the injured girl you brought home and rush back downstairs, just in time since you find her about to open the front door for "you".
You waste no time and start the merging process with her. You focus and take your pick, you want to keep your personality intact. You want her "comfort" with being a woman, her knowledge of female hygiene and everything else it means to be a woman. While you are at it you take all of her general knowledge, cause why not?