You are trying so hard to compartmentalize this mistake. I can imagine why, but I think there's something you don't understand here mate.

Your sister didn't make a "simple mistake" that only affects one element of her life. She made several huge, repeated mistakes that affected her life, her husband's life, your family's lives, and your life. She lied repeatedly to everyone, including you, and also forced you into an awful situation. At her age, she should know that asking you to lie for her is taking advantage of yoy, and she should've never asked this of you.

Basically she fucked up bad, and your family is responding in kind. Their response toward you might be a little overblown, but their response towards your sister seems fairly appropriate to me.

You obviously love your sister, which is wonderful, but she disrespected that love by putting you in this situation. You seem like a smart kid, and I'd guess that you're trying your best to preserve the version of your sister you think you know, but I also think you're about to learn a hard lesson: that sometimes people are not who we think they are, and our trust is misplaced.