Seemed like a work of surrealism
An entire extended family -- parents, siblings and TWO sets of grandparents all forget a teen's birthday? And the 16th of all birthdays? And it's not like they're a wild family? They're all supposed to be sturdy midwesterners.
And then when they finally figure it out they do virtually nothing to make up for it?
Finally, at the end, they go back to the guy's place and he has already prepared a birthday cake for her? How did he even find out, much less have time to do it?