In a promo video for an upcoming episode of Keeping Up With The Kardashians, the 35-year-old reveals the family only learned about Rob's plans to marry when stories began to appear on the internet.
"We wake up to all these pictures of my brother getting engaged," she says.
"How does our own brother not tell us?"
Rob, 29, proposed to Blac Chyna, 28, in May, just over two months after they confirmed their romance, which is believed to have begun at the start of the year. The engagement was announced via an Instagram photo of Chyna flashing her huge rock while seated next to a smiling Rob.
They celebrated the engagement at a strip club.
Chyna was previously a close friend to Kim. She has a son, King Cairo, with rapper Tyga, 26, who recently split from Rob and Kim's youngest sister, Kylie Jenner, 18.
Just after announcing their engagement, Rob and Chyna revealed they're expecting their first child together - and they're starring in a new reality TV series, which you can find out more about in this video:
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