I agree, her interest is in her son, not you. She doesn't know you from Adam by the sounds of it and really there is no context for her to do so if she doesn't know the circumstance.
Personally I am with Sourgirl on this one if I really felt I was just an add on I wouldn't go. If there were really good food, free booze, some good company other than the mum and your partner, then ya, I might go, but if I really felt like I didn't fit in and it was all just down right uncomfortable then I would find a place I am wanted.
As to the being out thing? I dunno, I don't attach any importance to it personally. What's my business with someone isn't others. Sometimes, where parents are concerned, it is more of a hassle than it is worth. I used to think that I should be a poly advocate all the way, now I'm kinda done with that in terms of being an activist in the out kind of way. I like when people find out and the stories behind them finding out. I like the mystery and wonder people have about why I spend time with certain people too. Really I figure that beyond it looking like a deep friendship most people don't know what the heck poly is and would prefer not to. On this note I would wonder if the mum is just doing her Christmas thing and getting about seeing her responsibility through. She might just appreciate not knowing.
On the other hand you could help her take a load off and help her out as much as you can. Speaking from the position of a mum, Christmas is a lot of fucking work and the burden is heavy for mum's. I hate it. If someone were to come to my house and offer to help, I would jump on it and give them kisses all over as a result. Something to think about there perhaps.
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