I tend to think the Madora Bay house was a holiday house as I have found evidence that BE’s parents are still connected to Gosnells area quite recently. Seems odd they would move to Mandurah then back to Gosnells.
I am speculating that it was their holiday house, their original block (in the city) was really large and the original house is now 2 and a separate house has also been built. I am guessing that they moved down to the beach house while the construction work happened. Also, I think there is at least another sister and possibly an older brother, though I have no idea of names or anything. If you look at google earth you can look back to I think 2000 and up to now.