Single crystals of (PbxSn1-x)(2)P-2(SeyS1-y)(6) with Se concentrations below and above the Lifshitz Point (y = 0.27) have been grown with the aim of studying the influence of Pb on the ferroelectric phase transitions. Their critical behavior has been analysed by means of the thermal diffusivity. It has been found for y = 0.2 with x = 0.1, 0.2, 0.3 that there is an increased frustration of the continuous phase transition compared to the case in which x = 0. This frustration is due to the weakening of the stereoactivity in the crystal because of the presence of Pb, as it prevents the formation of the Sn2+ lone pair electron cloud, responsible for the ferroelectric properties. The ferroelectric transition changes its character from second order to first order between x = 0.1 and x = 0.2 but without the apparition of a Tricritical Point. Instead, the critical exponents change suggest that the transition approaches the Mean Field universality class, which is in agreement with the predictions of the diluted BlumeEmery-Griffith model with randomness. Finally, for the case of x = 0.05, y = 0.4, 0.5, Pb addition also frustrates the transition with respect to the case of x = 0, reducing the critical temperature. At the same time, it induces the apparition of an intermediate incommensurate phase, between the paraelectric and the ferroelectric phases.