Complex sentence: This is a 1500 acre (6 km²) Canadian National Historic Site centred by the Thompson Mansion, which was the summer home of the Thompson family who owned the steamship lines on the Grand River when it was a commercial route.
Split into simpler sentences: This is a 1500 acre (6 km²) Canadian National Historic Site centred by the Thompson Mansion. This was the summer home of the Thompsons who owned the steamship lines on the Grand River when it was a commercial route.