I couldn't stop looking down my sister-in-laws shirt
Why did you look down her shirt?
I had to know if they were real.
That type of behavior isn't appropriate when you're in a committed relationship. It's more appropriate to focus your attention elsewhere. Your girlfriend and her sister will likely both be offended if they see you staring at her chest.
I don't think it's bad to look down someone shirt if needed.
Looking down a woman's shirt isn't appropriate. I urge you to focus your attention on other things when you're around your sister in law.