A colleague and I, early-career researchers, are planning to submit a research funding proposal. The research program we are envisioning would need a third PI with complementary expertise and resources. Of all the collaborators we might invite, one of the most qualified would be our former supervisor. (I worked under this supervisor as a doctoral researcher, and my colleague and I both worked under this supervisor as postdocs. All three of us are now at different institutions across Europe. My colleague and I have each already received a single-PI research grant without any involvement of the former supervisor.)
The funding program to which we are applying indicates that its purposes are only "to support excellent collaborative research projects" and "to make it easier for researchers to collaborate across borders", so including our former supervisor doesn't violate any explicitly stated guideline. However, I understand that "academic incest" is often a concern, for example when admitting graduate students or hiring faculty. Is it also a concern when evaluating research funding proposals? That is, is it possible/likely that reviewers would raise objections to a funding proposal that is submitted jointly by a professor and two of their former postdocs? If so, we'd rather invite a different collaborator to be the third PI and retool the research program around their particular skills and resources.