In a family of seven members, namely P1, P2, P3, P4, P5, P6, and P7, the following information is known:
• P4 is the sister of P7.
• P2 is the mother of P6's wife.
• P6 is the son-in-law of P3.
• P5 and P7 are the grandsons of P3.
• In the family, there are exactly: 2 fathers, 2 mothers, 2 brothers, 1 sister.
Based on the above information, what is the relationship between P1 and P4?