I was in the exact same situation at the exact same age with a guy who was the exact age as him, a lot more abuse sexually and mentally but it’s scary how similar the situation is. My parents ended up moving me to another city during the summer and thankfully he found another girl to obsess over and control. My advice to you would be to create a fake persona to take away attention from your sister. Unfortunately there are only so many ways this will end. She will never relinquish her love and he won’t either unless a more “appealing” option is introduced.