When Hallie is in London pretending to be Annie and has a sip of wine, she says she is partial to California wine or grapes... Wouldn't the mother or grandfather pick up on the fact that she said she's drunken California wine, when she knows her daughter and the father live in California?
I take it you've never seen a well-stocked wine cellar or ever been to summer camp before. The twins' mother used to live in the USA so I would expect her to include wines from the USA in her wine cellar. Also, I've been to summer camp and some of the older kids do sneek in alcohol into the camps (Wine, being cheap, seems to be brought in more then any other drink). It does seem reasonable that Annie would have sampled California wine before and a comment like her being partial to California wine might be taken as her pretending to be more sophisticated then she really is. Also, different wines have different flavours, so, for Annie to mention the first more pleasant tasting wine (for her)that pops into her head then the one she is sampling at the time would make sense.
How many times, as children, have we done something that we hoped and thought our parents didn't know about, just to find out later that they knew about it all the time? Raising a child around a well-stocked wine cellar and having open bottles of wine in the house, you have to expect a child to at least sample something forbidden once in a while. To me, this not being commented on at that time isn't a slip-up, just a parent accepting life going on.