The cousins are all private. They should be there if your are logged in [but I assume this is not your tree]. If you point, for example, with your cursor to one of the mother’s siblings in the side bar you will see her children all listed as “Private”.
Sorry, that was all wrong 🙁
The problem is that the “cousin’s” tab should not be displayed when not logged in. If not logged in then the number of cousins can always be wrong because of privacy, so becomes nonsense. Therefore I always recommend it is only visible to members or above.
In the current version of kiwitrees it is not possible to force that. But in the next release I can make members and above the only options, which I will try and remember to do.