Miley Cyrus' Brother Banging Patrick Schwarzenegger's Sister!

If JFK is watching ... here's your New Frontier ... two of your niece's children are banging the offspring of the dude who sang "Achy Breaky Heart."

We've learned Miley Cyrus is not the only one getting a piece of the Kennedys ... her brother Braison is dating boyfriend Patrick Schwarzenegger's sis, Christina.

Miley and Patrick have been tight since November. We now know a month later 20-year-old Braison began hooking up with 23-year-old Christina.

So here's the conundrum ... if both couples get married, Braison will be Miley's brother-in-law and brother, while Christina will be Patrick's sister-in-law and sister.

Camelot meets PBR ... awesome.