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the nearest i can see is grandstanding as it's done with specific motive - but it doesn't account for the reason they do it & the attitude after they've done it.
It's not just showing off or just flashing cash, it's the weird belief that somehow makes them better family than any others, when they do hardly anything practical day to day, hardly any actual quality time, they just want to have a nice time themselves a few times a year & expect a round of applause forever more for it.
They also think it gives them right to laud it over & even abuse others who dont/cant do the same.
it's possibly too long & too specific a circumstance for one word - some good ones for the showing off bit though.