Step 1: Lay out the generations.
Seven family members P1 to P7 travel together. We sort them into grandparents, parents and the youngest set using the clues.
Step 2: Anchor the grandparent.
P5 and P7 are grandsons of P3, so P3 is a grandparent and P5, P7 belong to the youngest generation.
Step 3: Place the middle generation.
P6 is the son-in-law of P3, so P6 married a daughter of P3. P2 is the mother of P6's wife, which places P2 in the grandparent generation alongside P3.
Step 4: Identify P4.
P4 is the sister of P7, so P4 is also a grandchild of P3, sitting in the youngest generation with P5 and P7.
Step 5: Find P1's role.
The remaining adult who is a parent of the youngest set must be P6's wife, the daughter of P3 and mother of P5, P7 and P4. The only label left for that mother is P1.
Step 6: Read off the relation.
P1 is the mother and P4 is her daughter, so P1 and P4 are mother and daughter, matching option 1.
\[ \boxed{\text{Mother - Daughter}} \]