I have seen this before and will try to explain how it happens.
I have an uncle with two children, they married siblings and I have the parents and marriage information of these siblings in my database.
I export a GEDCOM of my Grandfather's descendants and their spouses which will include these siblings but not their parents. But they are still linked as a family by that marriage information.
When the GEDCOM file is imported to a new database, thar marriage information shows up with no husband or wife. I can find these siblings in the familoy view as children, with blakns for parents, but that marriage date and place appear in the information area. ------------------
So, I doubt that you really want to get rid of it. It can be done, if you can find these parentless siblings and delete their family links. But there will be no way to tell that they are siblings anymore.
P.S. Finding them isn't all that easy either, unless you know who they might be. The children aren't listed in the “people without parents” list, and the Marriages list only shows the date and place, no names.