I found my girlfriend's sister's hidden pictures, and I can't stop looking.
What makes you want to look at them?
She's very attractive.
You shouldn't be looking at someone's private photos, those are for that person's eyes only. Especially if it's your gilfriend's sister.
It's not like I'm doing anything with her pictures. Just feels nice to look.
You're right that there's nothing wrong with just looking, but she didn't want you to see these and your girlfriend doesn't want you to look at them. In that way, you could be hurting two people by looking.