Blurred Lines Legal Fight Flames UP!

Two of Marvin Gaye's children aren't buying Robin Thicke's claim that "Blurred Lines" isn't a rip-off of the their legendary dad's music -- and are filing a counter suit.

Robin filed a pre-emptive suit in August in an attempt to stop the family from blocking the sales of "Blurred Lines." But now TMZ has acquired legal documents that Nona and Frankie Christian filed against him today (Wednesday). They point to interviews with Billboard and GQ in which Robin says Marvin's "Got to Give It Up" was one of his fave songs and that he was hoping to make something with a similar groove.

The Gaye suit further claims that Robin's song "Love After War" is a copy of the Marvin hit "After the Dance."

The family wants all the profits from "Blurred Lines" -- which would be a lot, since the song was at number-one for 12 weeks this summer.