$Fe ^{3+}$ ion is more stable due to its half-filled $3 d ^5$ electron configuration. As half filled and completely filled shells are more stable $Fe ^{3+}$ ion is more stable than $Fe ^{2+}$

The $Sn ^{2+}$ ions convert to $Sn ^{4+}$ ions because the tin$(IV)$ oxidation state is more stable than tin$(II)$.

In non-transition elements of p-block, lower oxidation state is more stable due to inert pair effect e.g., $Pb ^{2+}$ is more stable than $Pb ^{4+}$