Personally I love wish lists. I think they can be used in a lot of different ways--to get exactly what a tricky recipient wants, to inspire an original gift, or not consulted at all of course.
In the OP's case, it sounds like the family insists they give each other wish lists. So, no problems with people feeling resentful about receiving them from others, or else we've got a really conflicted group of people!
Then, they buy things that AREN'T on the list. Maybe they're just using the lists for inspiration. Sometimes, especially with the OP, these things miss the mark. The OP responds graciously with words, but her immediate expressions show her dismay, and the givers sometimes jump on that, or demand she respond with greater enthusiasm than is reasonable. This is the situation I'm getting from the OP, please someone correct me if I've got it wrong.
Ways to hide/manipulate facial expressions that I can think of:
--Practice in a mirror beforehand. Similar to practicing saying things in advance so they sound more natural.
--Assume you will be horribly disappointed with the gift before you open it. Then, when you open it and it's not quite that
bad, your happy/relieved expression will be more genuine.
--Say more when you open the gift. Maybe start by admiring the packaging before you even open it. Then, just keep babbling whatever halfway true compliments come to mind, even as you're unwrapping it. Hopefully this will distract people from your actual facial expression.
--Suddenly cough or sneeze as you see what the gift is, so any weird expression will be attributed to that. Even better if you can use a tissue to partially cover your face until you recover.
Some of these sound kind of convoluted, I grant you. But, I totally understand wanting
to be polite, and being undermined by immediate reactions you can't always help. And I think these are some reasonable ways around that.
FWIW, my problem has generally been the opposite, that I have trouble expressing the enthusiasm I genuinely feel. So I've gone the reverse route, practicing expressions, tones, and words that convey my (genuine) enthusiasm with a gift. I've found this has also come in handy with gifts I don't
feel genuine enthusiasm for.