Unless you’re a parent/guardian and your child is very young, keep out of other people’s emails — and then, they should know that you will be checking their email.
Assuming your sister is old enough is old enough to date (is that what we’re talking about here?) then she’s old enough to manage her own affairs. Stay out of her email. And if she’s not an adult, then talk to your parents about it. You stay out.
The Wolf