AITA for Telling My Pregnant Sister the Truth About Her Husband Even Though It Destroyed Her Marriage?

 I’m 29F, and my older sister is 34F.

She’s always been more like a second mother to me than a sister. When our dad left, she helped my mom raise me. She worked part-time during college just so I could have school supplies and birthday gifts. I honestly worshipped her growing up.

So when she finally met someone who seemed to truly love her, I was happier than anyone.

Her husband was charming, successful, respectful — at least on the surface. They got married four years ago and recently announced they were expecting their first baby after struggling with fertility issues for years.

My sister was over the moon.

She cried holding the ultrasound photos.
My mom cried.
Even I cried.

It felt like life was finally rewarding her after everything she’d survived.

Then I ruined it.

Or at least that’s what my family says now.

About two months ago, I went out with coworkers to a rooftop restaurant downtown. Near the bar area, I saw a man who looked exactly like my brother-in-law.

At first I thought maybe it wasn’t him.

Then he kissed the woman sitting next to him.

Not a friendly kiss.
Not something explainable.

A full romantic kiss.

I felt sick instantly.

I actually hid behind one of the pillars because my brain genuinely couldn’t process what I was seeing. He was laughing, touching her hand, acting completely in love.

Like my pregnant sister didn’t even exist.

I took a photo because honestly I knew nobody would believe me otherwise.

For two days, I said nothing.

I barely slept.
I kept staring at the picture wondering if I should destroy it and pretend I never saw anything.

Because telling her would break her.

But staying silent felt like betraying her too.

Finally, I invited him to meet me privately.

I showed him the photo.

The second he saw it, his entire face changed.

He immediately started begging me not to tell her.

Not denying it.
Not explaining it.

Begging.

Apparently the affair had been going on for almost a year with a coworker. He swore he still loved my sister and claimed he planned to end it before the baby arrived.

Then he said something I’ll never forget:
“If you tell her now, the stress could hurt the pregnancy.”

That sentence messed with my head badly.

Because suddenly I felt responsible for protecting her emotionally AND physically.

He kept promising he’d end the affair and become the husband she deserved. He even cried.

And for one stupid moment…
I almost agreed to stay quiet.

But then I imagined my sister giving birth beside a man living a double life, and I knew I couldn’t do it.

So I told her everything.

I showed her the photo.

I have never seen heartbreak like that in my life.

She didn’t scream.
Didn’t cry immediately.

She just stared at the picture silently for almost a full minute before whispering:
“How long?”

That question shattered me because I didn’t know the answer yet.

When she confronted him, he confessed immediately.

Apparently he assumed I already told her everything.

The next few days were chaos.

She moved out temporarily.
My mom blamed me for telling her during pregnancy.
His family accused me of trying to “destroy a marriage.”
Even some relatives said I should’ve waited until after the baby was born because now she’s emotionally devastated while pregnant.

But here’s the worst part:

My sister miscarried two weeks later.

The doctors said stress alone probably didn’t directly cause it, but emotionally everyone connected the two events instantly.

Now his entire family openly blames me.

His mother literally told me:
“If you stayed quiet, my grandchild would still be alive.”

I think about those words every single night.

My sister hasn’t blamed me directly, but she barely talks to anyone anymore. Their marriage is over. She looks emotionally empty now.

And even though logically I know HE caused this by cheating… emotionally I still feel like my truth detonated a bomb inside her life.

So… AITA for telling my pregnant sister the truth about her cheating husband?