HP Questions Upon Re-reading: Book 1 Chapter 6

Why is Aunt Petunia staring at Harry when he mentioned going to Hogwarts through Platform Nine and Three-Quarters? Lily must have gone through the same journey for seven years with her family. Despite being jealous of Lily for able to attend the magical school (she tried to appeal to the headmaster of the time for enrolment), it is hard to assume Aunt Petunia would not see her sister off for studying and living in a remote place for nearly half a year (assuming Lily went home for Christmas holiday). Therefore, it feels a bit off for Aunt Petunia not knowing the secrets to Platform Nine and Three-Quarters.