Sister's Engagement Party Exposed a Fraud

AITA for ruining my sister's engagement party after learning the truth about her fiancé?

Hi Reddit, I’m F29 and I desperately need an outside perspective. My sister,

"Clara" (F32), had her big engagement party yesterday. She’s been with "Marcus" (M35) for

six months, and they are truly head over heels or so I thought. The Background

Clara and I have always been very close. She has been my biggest supporter.

Marcus entered her life when she was going through a tough time, and he seemed to

treat her like gold. He is successful, charming, and won over the entire family instantly.

I had a weird gut feeling from the start, but I thought I was just being overly