We propose to use WISH to examine whether a transition between different spiral antiferromagnetic structures in zero magnetic field is the key to tricriticality at finite fields in CoMnSi and NiMnGe. We seek to correlate the temperature evolution of the magnetic structure in both compounds with their different magnetic phase diagrams and with the observation (in CoMnSi) or absence (in NiMnGe) of large entropy changes at a magnetic tricritical point.