The amount of messed-up that would be in this “prank” would be so…messed up.
The girl seems like she was excited and happy over the proposal, which would later turn out to be disappointment, anger, and sadness when she finds out it was fake.
She would be embarrassed that he doesn’t want to marry her but she really wanted to. There’s nothing wrong with one partner being ready and the other not, but having that displayed as a mockery is embarrassing.
She didn’t recognize that it was his twin brother, which is troubling. Even if she was unaware that he had a twin, I would expect her to pick up on some flags that it wasn’t her partner: 1) different mannerisms, 2) very limited knowledge of shared experiences and her particular tastes, 3) no jointly understood scripts for showing affection, and more.
When she is legitimately proposed to, she will have at least some considerable doubt that it’s real and prevent her from engaging safely and freely, thus damaging the real proposal.
She will now have to decide if she’s okay with marrying into a family that pulls “pranks” like that. Also, considering that her partner’s brother was fine with doing that, she may wonder if she is missing some serious flags with her partner.
Note: Please excuse me if I’m being too judgemental, picky, or similar. I found my cat of 3 years dead today, so I’m definitely more vulnerable and emotional.