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Meghan Markle's half-sister says England's new princess-to-be has no right to tell her when to talk, or what to say -- 'cause this is 'Merica, damnit!!! Yes ... seriously.

We spoke with Samantha Markle a few days ahead of Meghan's wedding with Prince Harry -- amidst a ton of family drama brewing among the Markles -- and she tells us she doesn't care if her sis wants her to refrain from blabbing to the media as much as she has.

Samantha insists she's not bad-mouthing Meghan ... she's just defending herself. And besides, Samantha says whatever censorship laws the U.K. have on the books don't apply to her across the pond ... even if Meg's now a big deal there.

You gotta hear her for yourself -- she's certainly got her opinions of Meghan.