As fans worldwide tuned in to see American actress Meghan Markle's extravagant wedding to Prince Harry, there was one person in particular who clearly wasn't impressed with all the fanfare — Kate Middleton!

According to a royal family insider, Middleton — who just gave birth to her third child, Prince Louis of Cambridge — was seething with jealousy.

"Kate was incredibly bothered because of how much more over-the-top Meghan and Harry's wedding was compared to her wedding to William," the source told RadarOnline.com exclusively.

"In terms of money, it also seemed to Kate that much more had been spent on this wedding than her own, which made her extremely upset," continued the insider.

It doesn't help that Middleton felt Markle snubbed her younger sibling, Pippa Middleton, by omitting her from the guest list, even though she ended up with an invite to the event of the year after all.

And though Markle, 36, is incredibly poised, she still has a lot to learn — much to the chagrin of Kate, also 36.

"As much as everyone wants them to be close, Kate does not have the time to teach her sister-in-law how to be a proper lady as the Queen wants her to do," said the source.

"Kate does not think that Meghan deserves to be a royal, and believes that she is in it for the wrong reasons."

Do you think that Meghan's wedding was more extravagant that Kate's 2011 ceremony? Sound off in the comments below.

We pay for juicy info! Do you have a story for RadarOnline.com? Email us at tips@radaronline.com, or call us at (866) ON-RADAR (667-2327) any time, day or night.