Miley Cyrus' Brother Banging Patrick's Sister!

If JFK is watching ... here's your New Frontier ... two of your nieces children are banging the siblings of a 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.