What's it like finding out your boyfriend/girlfriend is really your half sibling?

Not me but my friend's case. She was very much in love with her boyfriend of 2 years and both even talked about getting married after finishing college and forming a family.

She was raised by a single mother. Turns out her mother had a long-term affair with a married man and my friend was the product of it. The wife divorced him upon discovering the infidelity. Just recently both a DNA and paternity tests were done and it was positive. She's just as devastated as her brother. They obviously can't ever be in a romantic relationship. They're still grieving the loss of that relationship. I would like to place myself in their shoes for a moment, what would that feel like? To find out the person you love so much, been intimate with and want to marry one day is your half sibling?

What's it like finding out your boyfriend/girlfriend is really your half sibling?
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