Beyonce -- Second Banana At Mom's Birthday Bash

Beyonce and her sis Solange are shutting down a fancy restaurant in New Orleans Saturday night to throw their mom Tina an epic 60th birthday party -- we're told costing at LEAST $100,000 -- but the hilarious part ... Bey got second-billing on the invite!

It's a black tie masquerade-themed event, and Beyonce and Solange are hosting it at the pricey Muriel's Jackson Square in the Big Easy (we hear the turtle soup is beyond).

And under the category "Charity begins at home" -- Beyonce allowed Solange to get TOP BILLING!