I agree with everyone, it is a very delicate situation not a party - more the merrier.
If i were in your place, i would firmly stick to my guns, no way would i be told that someone was coming who had only met my mum once, absolutely no way and if this niece - who you have never clapped eyes on - insists, tell her you have decided and thats that. Well, maybe not as stern as that, but i would certainly make it known that your plans have been made.
Awkward it may be BUT it isnt as if you're in regular contact with her.
I hope all works out for you, you shouldn't have all this anguish to contend with.
