Will doing Masters degree help me get into another masters degree?
Presumably a higher relevant qualification ---or an equivalent qualification that is more recent and with better performance--- will be more valuable than an older qualification with poorer performance.  Additionally, having a Masters degree in statistics (hopefully with good performance) would be of relevant value to entry into sociology or political science.  As to whether it is necessary, who can say.  Perhaps the best option is to apply to both ---i.e., apply to the top schools for immediate entry, and also apply to the MSc in Statistics as a fall-back.  If you are not accepted for the former, you can still do the latter and then re-apply more strongly later.