What type of girl is your sister? if it is a real issue, and you are close enough, would you be able to bring up the inyan in conversation with her and discuss it ? (without letting her know that you read your email)
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.