We know the following relationships from the problem:
- M has a son Q and a daughter R.
- M's wife is E.
- E is the mother of P and daughter-in-law of M.
Since E is the mother of P, P is the child of E. Additionally, since E is the daughter-in-law of M (because E is married to M), P must be the grandchild of M.
Thus, P is related to M as the grandchild.