The 2015 Nurburgring winner had taken over the #911 Manthey Porsche from Kevin Estre and was battling for third with his younger brother's Phoenix Audi on lap 23 when they made side-to-side contact in the fast sequence of bends approaching the Hohenrain chicane.
The touch between the Audi's left rear and Porsche's right front caused Laurens Vanthoor to spin and make heavy contact with the barriers.
Dries Vanthoor continued unscathed and went on to secure his second victory in the 50th edition of the German endurance classic, together with Kelvin van der Linde, Frederic Vervisch and Robin Frijns, after a late rain shower turned tyre choice into a lottery.
Writing on social media on Sunday, Laurens Vanthoor explained that the desire to beat his brother meant "emotions took over and my brain stopped working".
"What happened yesterday was a nightmare," said the Belgian, who is racing in the DTM this year with SSR Performance.
"I know I’m smarter, I know you don’t take that amount of risk in a 24h race.
"It’s painful to be this open in public but the reason behind is that I was racing my brother. My emotions took over and my brain stopped working.
"We’ve been competing against each other since we were kids, never backing off and rather breaking a leg then [sic] loosing to each other.
"Unfortunately this competitiveness took the upper hand yesterday. I regret it badly, it was unprofessional.
"I have absolutely no hard feelings towards Dries. In the contrary, I’m a very proud brother. In the future we will have each others' back instead of fighting each other.
"Today was a black day in my career. I will regret it for a very long time but that’s how life works."
The best-placed Porsche after a disappointing 24 Hours for the Weissach marque was Falken Motorsports' example in ninth.
Porsche 911 GT3 R project manager Sebastian Golz said: "We were plagued by bad luck on Saturday and Sunday. We lost six cars in accidents, most of which were involved in collisions through no fault of their own.
"We very clearly witnessed just how strong our GT3 customer teams and our cars were in the race from the top positions of Manthey and Toksport WRT, which makes it all the more disappointing that we weren’t able to turn our competitiveness into a decent result.”
BMW "could have done significantly better"
Another marque left disappointed on Sunday was BMW, which was expected to fare well on the debut of its new M4 GT3 after the Rowe cars had shone in qualifying.
But only one of its cars made the finish, with the #102 Walkenhorst Motorsport car 26 laps down in 15th place after numerous contenders hit trouble.
Early contact for Nick Yelloly put the #99 Rowe entry out, fellow Briton Dan Harper was delayed by contact in the RMG-run Junior Team car that was later crashed by Max Hesse, and a problem with the front-right suspension during the night caused Sheldon van der Linde to crash the #98 Rowe machine.
The #20 Schubert entry of Jesse Krohn, Alexander Sims, Jens Klingmann and Niklas Krutten was set to finish fourth when it retired in the final hour due to an overheating engine.
BMW Motorsport boss Andreas Roos said: “Sadly, that was a very disappointing race for us and we could have done significantly better. Our expectations were high after the good, intensive preparations saw us show good pace and record some top results.
"It is particularly galling that we lost two cars early on due to our own errors. We allowed ourselves to be influenced by the hectic events on the track.
"The very positive aspect is that our new BMW M4 GT3 has shown that it has the potential to win this race, being right up there in qualifying and in the race."
