A source told RadarOnline: ''Kourtney has already seen Kim have an over-the-top wedding with Kris Humphries and now she's watching her sister turn her Kanye West date into another big affair and she's getting mad that her own wedding is going to be overshadowed.
''She wants her wedding to be just as spectacular as Kim's.''
Kourtney and Scott - who have Mason, four, and one-year-old daughter Penelope together - are reportedly keen to wed in Mexico, while Kim, 33, and Kanye, 36, who have eight-month-old daughter North together, are getting hitched in Paris, France.
The source added: ''For a really long time Kourtney has felt like she is living in Kim's shadow but she doesn't want her wedding to be second to Kim's third trip down the aisle.
''Everyone knows that Kim is the most popular sister and Kourtney really wanted her wedding to be special, so she's not planning to let Kim steal her spotlight now.''
Meanwhile, the couple - who have been together for over seven years - are planning an intimate wedding and will splash out $100,000 on round trip private jet flights for their loved ones, as well as $30,000 a night on accommodation.
The BBC drama starring Aidan Turner returns to BBC One on September 4th.