The grandson of designer-to-the-stars Oleg Cassini is suing to force the sale of the fashion great’s $15 million townhouse near Gramercy Park — the latest salvo in a tortured estate battle.

Since the designer’s death from a stroke in 2006, Cassini’s family with first wife, actress Gene Tierney, have fought with his widow, Marianne Nestor Cassini, over his estimated $55 million estate.

Cassini’s only surviving child, daughter Christina, is entitled to half of the fortune under the original Cassini-Tierney 1953 divorce decree, one judge found, along with $1 million left to her directly in Cassini’s will, judges have found.

But to this day, barely anything has been paid — except for the proceeds of the auction of two pieces of artwork, according to court papers.

In 2014, a surrogate’s judge in Nassau County removed the widow as executor of Cassini’s estate, citing multiple examples of mismanagement, misconduct and outright hostility to Christina.

But a year later, Christina, who suffered ovarian cancer, died in poverty in Paris.

Now, Christina’s son, Alexandre Cassini Belmont, who is based in Ibiza, Spain, is joining the estate battle.

In court papers filed Thursday in Nassau Supreme Court, Alexandre demands that the neo-Gothic, four-story, single-family townhouse be liquidated via sheriff’s sale, with $768,615.58 of the proceeds going directly to his late mother’s estate.