Flexible microelectromechanical (MEMS) devices are poised to scaffold technological innovations in the fields of wearable sensors, implantable health monitoring systems and touchless human-machine interaction. Here, we report the magnetoelectric properties of cost-effective and room-temperature sensitive 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3/Ni50Mn35In15 (PMN-PT/FSMA) multiferroic heterostructure integrated on flexible stainless steel substrate via RF/DC magnetron sputtering technique. The growth of the pure perovskite phase of PMN-PT without any pyrochlore impurity is confirmed by the dominant (002) orientation of the tetragonal PMN-PT. The double logarithmic plot of current density with electric field validates an Ohmic conduction mechanism with low leakage current density of ~10-6 A/cm2. The anomaly observed in temperature-dependent dielectric and ferroelectric characteristics of the heterostructure overlap with the martensite transformation regime of the bottom Ni-Mn-In (FSMA) layer. The PMN-PT/Ni-Mn-In multiferroic heterostructure exhibits a significant magnetodielectric effect of ~3% at 500 Oe and can be used as an ultra-sensitive room-temperature magnetic field sensor. These results have been explained by an analytical model based on strain-mediated magnetoelectric coupling between interfacially coupled PMN-PT and Ni-Mn-In layers of the multiferroic heterostructure. Furthermore, the excellent retention of magnetodielectric response up to 200 bending cycles enhances its applicability towards flexible MEMS devices. Such PMN-PT based multiferroic heterostructures grown over the flexible substrate can be a potential candidate for piezo MEMS applications.