UPDATE: My mom [55F] invited my[21M] girlfriend [21F] over for holiday dinners this year...then un-invited her after the first one without any reason, other than it's uncomfortable having non-family there?

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Original tl;dr: Girlfriend from France invited to Thanksgiving and Christmas, then uninvited from Christmas after Thanksgiving...family insists she didn't do anything wrong but that they feel uncomfortable having someone who isn't family at our holiday gatherings. I'm not sure how to tell Sandrine or what to do.
I really appreciate everyone's response to my post! I was starting to feel a little crazy and felt like I was being selfish not wanting to spend Christmas with my family, so I appreciate having other people validate the situation.
Basically, I called my mom and asked her what the deal really was. I said that she was the one who wanted Sandrine over, and it doesn't make sense to un-invite her and have treated her coldly at Thanksgiving. I told her that it not only embarrassed me, but was flat out rude to Sandrine. She maintains her story of "it's uncomfortable having non-family" at holiday events. It seems like most people think my sisters convinced my mom to un-invite her because they were uncomfortable - for multiple reasons, either that they don't like her or are self conscious about being single - and I think that if that's the case, my mom is sticking to it and isn't budging. I asked if it had anything to do with them and her response was: "Of course not!" Lol.
I told her I wanted to spend the holiday with Sandrine and that she was putting me in a really awful position having to figure out how to break this bad news to her. My mom asked if we can just talk about this again sooner to the holiday and if we could drop the topic for now. Whenever she knows I'm right about something she tries to delay the actual conversation so I said absolutely not because if we're not coming over for Christmas, then I need to know now so that I have time to tell Sandrine and also time to plan something fun for us to do. She said I was being "unreasonable". My sisters both texted me in our group chat - a couple hours after my phone call with my mom - and asked why I'd choose my girlfriend over them. The whole "blood is thicker than water" thing. I'm not responding. The sad part about this is that Sandrine had texted me a screenshot of the presents she got my sisters asking if I thought they would like them (this was right before my mom told me she was un-inviting her from Christmas). I had forgotten about that at the time of writing my original post. So now in addition to telling her we aren't spending the holidays with my family, I need to figure out how to help her return the gifts.
My issue is that I feel like it might make the most sense to say that I'm having some trouble with my mom and sisters right now (which is true) and I would rather not see them at the holidays and would rather spend time with her. This would avoid having to involve her in the reasoning for not going. She wouldn't feel bad about them "not liking her" and she wouldn't feel guilty for being the reason I'm not spending the day with them, which were two things a lot of people brought up in their comments.
The thing is, she will still want to send them the gifts she bought. She'll probably want to mail them because she won't want to look rude not giving them gifts. Does anyone have any tips on this end? Not only do they not deserve her gifts, I don't want her spending money on people who were so thoughtless and rude to her.
I'm looking into what we can do for the holidays and really appreciate the suggestions! We're on the west coast so unfortunately the New York trip is not feasible for us, but I've still got a few other good ideas :)

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tl;dr: Mom won't budge and still maintains same answer. I've figured out a few ideas of what Sandrine and I can do over the holidays, but still haven't figured out the best way to tell her. If I tell her the truth, it will offend/hurt her. But if I'm not truthful, she will not know the reality of the situation and will still (innocently) want to give them their Christmas gifts.


Source.

Your mom sounds trashy....