Can anyone explain

I watches the first half of this film a year ago and then had to take it back to redbox, not having time to finish it because I was interrupted in the middle. I didn't want to get it back late, it was already a day late, and I was kinda bored with it so I said eff it.

Now I am watching it pretty much where I left off, I didn't wanna watch the first 45 mins or so again because it's pretty slow, I know the jist of it, whatever.

And since it's been a year someone spoiled the twist for me. So my question is, knowing the ending, something doesn't make sense.

And I am sure it's because I am too lazy to watch the beginning again,

How do the kids not know what their own grandparents look like again? I know the mom had a rift with them. But what spurred the trip initially, and was the the explanation for the kids going, but never seeing what they even looked like? If the mom hated the parents so much to not even have the kids see pics of them why were they going to visit?