Kourtney Kardashian is reportedly jealous of Kim.
The 34-year-old reality star has kids Mason and Penelope with boyfriend Scott Disick.
Kim and her fiancé Kanye West welcomed their daughter North in June last year and are now engaged to be married.
The couple are currently renovating their luxury Bel Air home and enjoying a lavish lifestyle, which is said to be aggravating Kourtney.
"Kim's extreme wealth is causing resentment. Kourtney thinks Kim can be tight sometimes, and has been making digs. She gets jealous when Kim talks about her private jets and says she rarely wears clothes twice," a source told British magazine Closer.
"The tension started when Kourtney was house hunting. It might look as if she and Scott earn a lot of money, but they're not really as well off as Kim. So when Kim boasts about her new home, how much her engagement ring is worth, and her wedding plans - the budget for the flowers is £1million - it really annoys her."
It appears it isn't just Kim and Kourtney who are dealing with issues at the moment. The siblings’ half-sisters Kendall and Kylie Jenner are also believed to be on different levels, with 16-year-old Kylie resenting the 18-year-old.
Khloé - the younger sister of Kourtney and Kim - is said to be helping Kylie through this difficult stage.
"Kylie's sometimes jealous of Kendall and struggles when people compare them. Kylie feels like she's living in her shadow," the source added.
"She lashes out and says b***hy things to Kendall because she feels abandoned... Khloé knows what it's like to feel left out and is being really supportive."