This is bad. She may do something suicidal if you leave her alone. Which is why oniichan needs to restrain her. But not just restrain, make sure it's specifically imouto's hands and feet. Why? She can still cause harm to herself if her hands and legs are free. Now, you won't want to make it uncomfortable so make sure you restrain her on a bed. As for her emotions, you would need to kiss her once in a while.
Summary: Oniichan should tie up imouta onto the bed and make out with her.