Rob Kardashian and new fiancée Blac Chyna may be head over heels and heading down the aisle soon, but sources say some of his sisters are flat out livid.
"The family is really not into the engagement," a source tells PEOPLE in this week's issue.
For more on Rob Kardashian and Blac Chyna's joyful, drama-filled engagement, pick up this week's issue of PEOPLE, on stands Friday
When it comes to who's most upset by his impending nuptials, sisters Khloé and Kylie are said to lead the pack.
"Khloé and Kylie think Chyna is desperate, thirsty and playing with Rob's head," says the source. "They're also concerned she is after his money and that she is dying to stay relevant and get back at Kylie by being with Rob."
Kylie – who's currently dating rapper Tyga, Chyna's ex-fiancé and father of her 3-year-old son King Cairo – stands at the center of the controversy. Last year big sister Khloé came to her defense when rumors spread that she'd stolen Tyga from Chyna while still a minor.
And despite Chyna's success at getting Rob healthy again, the sisters' grudge still stands.
"As far as they're concerned," says the source, "Blac Chyna will never be a Kardashian."
Reporting by MELODY CHIU