When siblings contribute to a research paper, do they need to declare their relationship in the competing interest form?
There is already an assumption that authors working together have some level of positive relationship - whether colleagues, advisor/advisee, or relatives. These things need not be declared, because they don't influence the content of the work besides all the authors standing by it (which is a minimum requirement for even submitting the paper).
If a sibling has a stake in a company selling widgets and the paper is about how widgets are superior to doodads, then this likely should be reported because the paper could be influenced by a sibling's desire to help their relative's business.