Go to the scene where the Mom and Hallie are packing for San Francisco. Hallie enters the room, and tells mom that dad called. Then mom says, "Why don't you go collect the tickets from your grandma-father."
She was talking like that because she was so anxious about meeting her ex-husband again that she was having trouble thinking clearly. It's deliberate to show her anxiety.
She isn't drunk, she's nervous. Elizabeth is a little apprehensive about seeing her ex-husband after 11 years. Because she is nervous, she accidentally calls him "grandmother", then she catches herself. This isn't a slip-up.