Is it normal to be attracted to your girlfriend's sister?
Mostly.Normal for most people
Yes, it can happen: attraction isn’t something you choose, even when the person is off-limits. What matters is how you handle it and whether you respect your girlfriend and her sister’s boundaries.
You can notice that someone is attractive without wanting to hurt anyone or act on it. Sometimes the feeling passes; sometimes it sticks around, especially if you spend a lot of time together.
The important part is not turning it into flirting, secret messages, or a confession that puts the burden on them. Keep some boundaries, and think about what you want from your relationship with your girlfriend before doing anything that could damage it.