Totally late to this convo, but it struck a chord so I'm gonna pitch in a little.
My ex SIL was very much like your relative (except she always had 10-12 people with her). Too many instances to recount. Long story short, I let this woman ruin every single holiday and get together for years. I would be bitter and resentful for days before and afterwards. Given the shit she did (and didn't do), I think I was perfectly justified in being bitter. But I finally figured out the only person paying any price WAS ME!!!! She didn't give a shit. She came and had a great time and never thought twice about it.
In addition to spending days being pissed, I wasted countless hours planning all my wonderful passive agressive retributions in my head too. Good God! The time I wasted on this situation. But you know what? If she's so clueless as to not see anything wrong with what she was doing, my passive aggressive shit probably wasn't gonna have any effect either. (Except to make me feel better in the short term, but then again not really, cause I'm being this petty person and that's just not who I want to spend my time being. Leads to kind of a circle jerk if you ask me.)
So...my advice is to just move on. Accept she's gonna do this and don't let it ruin your time. Ultimately you're not gonna change her. You can only change your reaction to her. Unfortunately I didn't have this epiphany until way late in the game. I'm sorry I didn't have it earlier cause once I figured it out, I actually started enjoying myself. Which is really what these things should be about.
(On a side note, I would encourage you to look at WHY this bothers you so much. I know when I finally figured out why my SIL bothered me - besides the obvious irritation at total lack of common courtesy - that opened a big door for me in terms of how I deal with, and view, people in general. But that just was an added bonus for me. LOL!)
Good luck to you!