Petunia was accepted to the top girls' academy in the country— cost hundreds of pounds to send her, and for a year, that was all her parents talked about. She was their favorite child… for a while.
But when Lily got her letter from Hogwarts, everyone in the family ignored Petunia. Of course, they couldn't talk about Hogwarts openly, but they discussed it as often as they could among themselves. They praised and adored Lily, slowly forgetting their other daughter.
When Petunia married, her parents went to Lily's wedding instead. So when her parents died, Petunia merely "forgot" the funeral.