You wanted to find a way to punish your sister for what she did to Amelia, and seeing her next to Amelia's mom just gave you an idea. Why not merge them ?
You decide that you want your sister to be part of Amelia's mom rather than Amelia's mom being part of your sister. You also decide that she shouldn't get any younger thanks to merging with your sister, to avoid Amelia suddenly becoming to young for you to date one day.
As they merge you feel your body start to change a little bit and your memories, or rather Amelia's change to match the changes the device has caused.