Inclusive fitness has been under intense scrutiny in recent years, with many critics claiming the framework leads to incorrect predictions. We consider one particularly influential heuristic for estimating inclusive fitness in the context of the very case that motivated reliance on it to begin with: the Sir Philip Sidney signalling game played with relatives. Using a neighbour-modulated fitness model, we show when and why this heuristic is problematic. We argue that reliance on the heuristic rests on a misunderstanding of what (...) it means for two organisms to be related and perpetuates a mischaracterization of the role of the ‘relatedness’ parameter in inclusive fitness. _1_ Introduction _2_ Heuristic Inclusive Fitness _3_ The Sir Philip Sidney Game _4_ Model _5_ Results _6_ Conclusion Appendix. (shrink)