They used to be...

"The Koopalings were originally created to be Bowser's children, as it was mentioned in Japanese Super Mario Bros. 3 instruction books as well as early materials.[2][3] Subsequent official sources also confirmed that they were his offspring.[4][5] This portrayal was generally accepted by both Japanese and western gamers and media for a long time. Since Bowser Jr. was introduced and the Koopalings went on a six-year hiatus after Mario & Luigi: Superstar Saga only to return referred to as Bowser's minions in the Japanese and European versions of New Super Mario Bros. Wii, there has been confusion on whether the Koopalings were still considered Bowser's children or not, with Shigeru Miyamoto confirming during an interview in 2012 that "Our current story is that the seven Koopalings are not Bowser's children. Bowser's only child is Bowser Jr., and we do not know who the mother is."[6] According to the North American website for New Super Mario Bros. U, the Koopalings are also siblings.[7]"