Xpander is not the flagship, Xpander is the "module" version of the Matrix 12, in other words, without a keyboard.
The Matrix 6 or 6R is fairly similar to the Matrix 12, but with fewer oscillators (6 instead of 12).
The only similariity the Matrix 6 and Matrix 12 have is the name Matrix, the 6 is a completely different animal. The 12 has VCO's while the 6 has DCO's. The 12 has many different filter types, the 6 has one. The 12 has several dedicated controllers to change perameters and several large LED displays, the 6 had two incrumental buttons and one small LED display. The 12 was hand made in the factory in Los Angeles, the 6 was machine made in a factory in Japan and then shipped to Oberheim for final testing and distribution. I should know, I was the one who tested all those damn keyboards and racks back when they were new.
As far as lineage here is what I know:
Xpander came first, its a 6 voice synth with no keyboard. The Matrix 12 was released the following year and is basically two Xpanders with a keyboard.
The Matrix 6 family is made up of the 6 keyboard, the 6R rack version and the one space rack Matrix 1000.
Then their is the X
family which is made up of the OBX, OBXa, and the OB8. Their was also the OBSX which was a simplified non programmable OBXa. The OBMx is not related and is probably the last real Oberheim, the OB12 was made by Viscount and is a POS that is in name only an Oberheim.