why did Nick and Meredith sleep together?

It doesn't make sense with rest of story... Meredith is crazy but never seemed like the cheating type, especially when she doesn't really know what's going on with Jason in Fla...

Then on other hand.. why make Nick look like bad dude to his cousin who he loves?? Dude is awesome throughout the entire movie then she tells the waiter that one line...

Doesn't make sense..Rest of the movie I enjoyed.