Usher's Brother -- Baby Mama Demands More Cash ... I Know You're Rich!

Usher is already involved in a bitter custody fight with his ex-wife, Tameka Foster -- now TMZ has learned his name is being dragged into another one ... involving his half-brother.

According to court docs filed in Georgia, Usher's half-brother James Lackey (they have the same mother) is locked in a nasty court battle with his baby mama Martina. The two have gone back and forth with nasty allegations -- he claims she ran off with their kid to Michigan; she claims abuse -- but where Usher comes in is, of course, the money.

Lackey -- himself a record producer who has worked with the likes of BOB, Usher, Lloyd and many others -- claims the $1,300/month he's shelling out now is beyond his means.

Martina begs to differ, saying in her filings, "Given his opulent lifestyle, he has substantial income and assets available to him. He produces music with and for his brother Usher, he owns a recording studio, receives royalties, and owns a home and has several cars."