EXCLUSIVE

Paris Hilton's brother Barron is about to take Lindsay Lohan to her favorite place -- no, not the Chateau Marmont -- he's planning to take her ass to court!

We broke the story ... Barron Hilton told cops Ray LeMoine beat his face in at Lohan's request Friday morning -- and TMZ has learned the 24-year-old Hilton is now planning to file suit against LiLo and Ray.

Sources close to the Hilton's tell us Paris met up with a lawyer Saturday afternoon -- before threatening Lindsay on Instagram -- and hooked him up with Barron to get things going STAT!

As we previously reported ... Ray claims Lindsay is 100% innocent -- and says he only punched Barron after the Hilton family member got physical first.