Honestly, you are not alone and don't feel bad! That post almost could have been written by me.
I'm one with barely no maternal instinct and I go through the same thing when my family comes to visit, even though I love them to bits. I find it especially disruptive and hard to handle if they pop by unannounced, and I can't do the pretend play either. Don't even get me started on crying babies...
Everyone knows kids can be an absolute handful, especially when there's more than one of them at a time. It's understandable to not be able to handle the noise, especially if you have AS and sensory issues. It's just what kids can be like and you know you still love your nieces and nephew. It's just the likes of children you can't stand, and the activities that go along with them, like pretend play.
I don't think you're being selfish at all, but maybe when they come over you could just spend a little time with them doing something more comfortable for you. That way your mom should be happy that you've spent at least some time with them. How old are your nieces? You could set up a video game to play with them, or do a quiet activity like drawing. When you've had enough, just be firm and say that you have other things to do, or something along those lines (and hopefully they'll occupy themselves with something else). 
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Your neurodiverse (Aspie) score: 129 of 200
Your neurotypical (non-autistic) score: 100 of 200
You seem to have both neurodiverse and neurotypical traits
AQ: 39 / 50