I am surprised that your family has a Japanese car.
FYI: 宅 primarily means “a house”. Attaching お (a polite prefix) with 宅, the phrase means either “your family” or “your house”. We don’t precisely distinguish between “house” and “home” (or family members).
持っている「そうで」literally means “I heard that (your family has…)”. If you prefer a word-for-word translation, the Japanese sentence means “I heard that your family has a Japanese car, and I was surprised.”
You can also say:
お宅では日本車をお持ちとは、驚きました。(= I found that / I got aware that your family has a Japanese car, and I was surprised.)
The original Japanese sentence with ～そうで is preferred when you are not 100% sure whether the person actually has a Japanese car. My alternative sentence with ～とは implies that the speaker is very sure of the fact.