Nik Wallenda -- Family War for Grandma's House ... Harder Than Highwire Act

Famous tightrope walker Nik Wallenda is in a messy and public feud with another member of his famous family, and it's all over the home Grandma Wallenda left behind when she died.

Jenny Wallenda -- matriarch of the "Flying Wallendas" -- passed away last month in Sarasota, FL, and left her house to Nik. Family sources tell us the place is pretty beat up, and Nik's been working on it for years.

Now Nik's squaring off with his Aunt Tammy ... who'd been living in the house while taking care of Grandma Wallenda.

Ten days after granny passed, Tammy says she got a letter from Nik's attorney informing her she had 30 days to vacate the premises. We're told Nik has sunk $100k into repairing the house, and plans to turn it into a rental property. Problem is ... Aunt Tammy won't budge.

We're told Nik isn't putting Tammy out on the street -- he's offered her $1,000 for moving expenses, and her first month's rent on a new place. Tammy's turned him down, and instead she's raising money to hire a lawyer and fight Nik in court.

Nik tells us he's upset Tammy is taking the family feud public, adding ... "I prefer to handle it through proper channels."

And he thought crossing the Grand Canyon was hard.