Does a DS defend all planets in a shared system, or only those of the family that owns it?
Just the family who owns the defense station.
The Defense station is a big space station, that helps your family to defend against bigger opponents. 30% of every attacking enemy fleet in a system, where the defense station is stationed gets destroyed. Only the family leader is able to buy and move defense stations. Defense stations can only be moved to systems where your family owns at least 70% of all inhabited planets. If you own less than 25% of the planets in a system where you have a defense station, the defense station gets destroyed upon the next successful attack of an enemy against one of your family members.