'Big Bang Theory': Poet's Family Sues ... You Screwed Me Over Soft Kitty

The daughters of a poet claim the folks behind "The Big Bang Theory" pilfered their late mother's poem and didn't pay a penny.

Edith Newlin published the poem in 1937 ... it's called "Warm Kitty." Her daughters Margaret and Ellen say in 2007, Warner Bros. wanted to use the lyrics for a song in the show -- a song the cast sings to Sheldon when he's sick -- which includes the lyrics, "Soft kitty, warm kitty."

Margaret says the studio cut a deal with a music company to use the lyrics, but not her ... and she says she and her sister are the rightful owners.

What's worse ... she says the song is part of 'Big Bang' merchandising.

So the song on 'Big Bang' has been used for years and now she wants the studio and others to ante up. She also wants an order prohibiting the show from continuing to use the song.

We reached out to Warner Bros. ... so far, no word back.