Q's mother is sister of P and daughter of M. S is daughter of P and sister of T. How is M related to T?

S is daughter of P and sister of T. So, T is daughter of P. 

Now, the sister of P is the daughter of M. This means that P is also the daughter of M. Clearly, T is the granddaughter of M. 

So, M is the Grandfather or Grandmother of T.