Maybe because it was just the "clearing" of an inheritance dispute?

The brother says there was nothing like that, but there are written facts about the dispute and no contact.

Maybe the brother is just afraid, but maybe he is also just stupid and not aware of electronic verification possibilities (but would be hardly believable).

It could be an inside job and be presented as a semi-professional act (running out of bullets & time to kill the, in that case unimportant children and killing a passing witness - which could have been an important person, that´s why real inside jobs avoid unexpected collaterals), the combination of a RAF (who knows, if he is just a witness? Who else has seen him being overtaken by the other cyclist), nuclear scientist (too strange, that the dead "witness" got found behind the car - who would join freely such a situation he just arrived at?? He should have been shot with his bike on street whilst trying to flee) and a satellite expert (this was not a known place to spend your holiday afternoon (a plate also warned trespassing) and, according to the ongoing investigations they there not dressed or prepared for wandering, picknick or the like) is highly suspective.

Why was the car locked (RAF "had to" break window), but the older girl was outside, when the younger said they were all inside the car. Why did he drive backwards (regular reflex should be to drive towards the shooters, especially if this is also the "exit" of the parking - watch the pics, view also the blood stains, which show the placings of the bodies).

Irregularieties, but not unexplainable, but often the easiest answer is the suiting one: cheap wannabe professionals who lost control

One more proposal: he got lost in that road, whilst the turn around his elder daughter went to pee and suddenly evil Nazis of German descent showed up and killed the, to their eye foreign intruders.
I am a bit disappointed that this option didn´t already got consideration (evil Nazis are everywhere and are the worst people who have ever been on earth - I am a well educated German habitant, smile).
Sorry, but this was too attracting...