Not special snowflake as much as par for the course for this particular branch of the family . . .
Niece has a son who just turned 4 (a year younger than Babybartfast, who turns 5 in a few weeks). She was a single teen mom when she had him and she's made some so-so life choices since and long story short, we don't see her much now that Crazy SIL has run off to California. MIL only hears from her when she wants money (which is why we've stayed away). She's a reasonably nice young woman, but she's done practically no parenting, so her son is one of the worst-behaved children I've ever had the misfortune to share a restaurant with. (Followed closely by Crazy SIL, but I digress . . .)
So Niece had a birthday party for her son. We weren't invited, but MIL and SIL2 were, and apparently Niece wasn't all that subtle about expecting some very fancy presents. MIL "borrowed" Babybartfast for the afternoon so she and SIL2 could at least hang out with the kids at the party instead of drinking with Niece's friends the whole time. Babybartfast picked out a modestly-priced-but-fun present and by all accounts had a good time.
But when MIL and SIL2 came to return her, SIL2 mentioned how much Niece was looking forward to Babybartfast's birthday party.
Me: Uh . . . we're just having a small party here, for her friends. Not really intended for a big family thing.
SIL2: Oh! I didn't know that. I told her I didn't know what y'all were planning but I was sure she and [Son] would be invited.
Me: Not really. (With the unspoken suggestion "We weren't really planning for you tag along, either . . .")
So as far as I know, Niece has invited herself to Babybartfast's party. Luckily, SIL2 didn't know where or when we're having it, so it's not like Niece can just show up, and MIL (for once) knew enough not to go formally issuing invites on our behalf. I do expect MIL to show up, but telling her not to come would be like expecting the sun not to rise* so that's one I can live with. If Niece bothers to ask me herself, I'll have to explain. More likely, she'll ask MIL about the details, so I had to talk to MIL later and specifically tell her not to share the where/when.
And if all else fails, Niece will be surprised if she brings her son here and they are surrounded by the girliest "Princess party" I can throw together :-D
* Autocorrect changed that to "snot to rise."