What should I do with my sister and boyfriend?

your sister is wrong she is an adult, 24 years old she should know better to send her little sister to steal she put you in danger if your sister really had your back she would not threaten to tell on if you threaten her maybe she might not say anything but think about the loyalty and trust of this relationship the only way you guys can keep each other secret is threaten each other either reslove it without threats or tell your parents of both your wrongdoings