The magnetic and transport properties of bulk doped manganite, La 0.5-y Y y Sr 0.5 MnO 3 have been studied as a function of yttrium doping, y, for 0 ≤ y ≤ 0.25. In the absence of the yttrium this is a well studied half-doped manganite which is a metallic ferromagnet. The end members of this system (with y = 0 and 0.25) are found to be structurally different (rhombohedral and orthorhombic, respectively) due to the large distortions caused by the yttrium. These distortions make this series of compounds particularly interesting. The magnetization obtained in zero field cooled (ZFC) and field cooled (FC) studies show dramatic changes as the value of y is raised above 0.05. The magnetic behavior is further investigated by a study of the hysteresis loops, frequency dependent susceptibility and studies of the magnetic relaxation. The results are understood in terms of the average size of the cation on the A site, <r A >, and the variance, σ 2 (= ?y i 2 r i 2 - <r A > 2 ). The value of the variance, σ 2 , increases rapidly with doping, y, induces melting of the long range ferromagnetic ordering, exhibiting a stepwise variation of magnetization (metamagnetic like). Based on the present results a magnetic phase diagram has been drawn demonstrating different magnetic phase sequences with the variation of σ 2 .