Seven people of a family P1, P2, P3, P4, P5, P6 & P7 go on a picnic in a sports utility vehicle. 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. Two people in that group of 7 are fathers; two are brothers; and two are mothers, and one is a sister. What is the relationship between P1 and P4?