Gary's sister -- Felice Catena -- inherited the rights to Bruce's music after he passed away in 2006.

Now, she's filed a lawsuit in L.A. County Superior Court, claiming Capitol is shafting her ... paying only a teensy-weensy fraction of what she's owed on download proceeds for Knack hits like "My Sharona" and "Good Girls Don't."

According to the complaint, Capitol owes Catena not 12% ... not 25% ... but 50% of net receipts from sales of his music by iTunes, eMusic, Amazon, and others.

You see, back in the day -- recording contracts didn't contain provisions for digital downloads (obviously) ... so once iTunes came around, it wasn't clear whether to pay the artist their sale fee (around 12%-18%) or their licensing fee (around 50%).

Felice claims sales through digital distributors are actually licenses -- and she should be paid accordingly. Eminem infamously sued his record label for the same thing back in 2007 ... and won.