The German actress is close to her sibling Stefan and was devastated when he contracted the illness, which is transferred to humans from insect bites, a few years ago.
And the Inglourious Basterds star admits the situation is so bad, her brother is on a daily cocktail of drugs to keep the disease under control.
She tells Britain's Marie Claire magazine, "He's really ill right now. He's got Lyme disease. He got bitten three or four years ago and it attacked his nervous system.
"It's really a serious thing. He has to take antibiotics every day and can't work right now, poor thing."