As Life & Style reported earlier this week, Pippa Middleton's engagement to James Matthews has raised some concerns for older sister Kate Middleton — namely when it comes to the groom's younger brother, Spencer, 28.

"Spencer has slept his way around London and left a lengthy trail of women in his wake," an insider reveals. "The last thing [Kate] wants is for William's family to turn against her because of Spencer's actions."

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The 34-year-old mother-of-two is increasingly worried about Queen Elizabeth II's opinion, especially since the monarch already finds the Middletons "tacky," according to a confidant.

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"Pippa knows a lot of royal secrets and will have shared them with her fiancé," a source says. "Kate just hopes and prays James hasn't passed them on to his brother Spencer."

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’s best moments with the royal family.