Kate Middleton may be thrilled over her sister Pippa Middleton's engagement to hedge fund manager James Matthews, but the Duchess isn't happy about the black sheep in his family.

Sources tell Life & Style that the 34-year-old is worried about James' unpredictable younger brother Spencer Matthews, who has made quite a name for himself in the reality TV world.

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“Kate has been complaining to Pippa about Spencer slipping up. But James has promised that Spencer will keep quiet and Pippa believes her fiancé, so she thinks Kate should keep her opinions to herself,” an insider reveals to Life & Style.

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Not only has Spencer appeared on multiple reality shows — including a short stint on I'm a Celebrity…Get Me Out of Here! during which he was kicked off for steroid addiction — he's also claimed publicly to have bedded over 1,000 women.

Another source shares that Kate will do her best to avoid him at the wedding. "[She] will obviously be friendly, but she hopes she is not expected to pose for photos with Spencer," the source says, adding, "And William plans to ask that they be seated far from him.”

For more on Pippa’s engagement, pick up the latest issue of Life & Style, on newsstands now! And scroll through the gallery below for a look at Kate and Pippa’s sweetest moments together.