The quote in question is making the rounds now in quite heavy circulation as 'quite fitting' — and as internet 'nice quotes' usually go, it is unsourced.
If sourced at all, the references point to Wikiquote. That page lists as its source "As quoted in 'Surfing the Tao: A Revolution of Free Will (2004) by Angela V. Michaels".
A book title that does not inspire any confidence. That book also isn't available to me nor have I found a way to search its contents in any way. Except for the rather dubious description:
This is the heart wrenching story of a little girl who braves her mother's anger, rejection and abuse during her formative years. It is a story of America's shame–the dirty secret of child abuse that hides in homes all over the country.
Josef Mengele is perhaps the most notorious German concentration camp 'doctor'. As a German speaking nazi, this quote should be retrievable in German language, as a supposedly "original".
Of course it isn't that easy. No translated paraphrase I could think of shows up in any search. Even German language quote aggregators seem to only to list the English version, not even any direct or indirect (re-)translation.
Je mehr wir euch antun, desto weniger werdet ihr glauben können, dass wir es tatsächlich durchführen
A version almost singular in search results, if not for this one horror movie set-report from 2009.
A gBook search shows a similar picture: even the seemingly higher quality books that contain this quote do not show any reference for it. Being a lousy researcher that may all be my fault. But the constant lack of any references for that quote just amplifies suspicions.
One problem that is illustrated here may be that quite simply not everything is 'on the net'. Another problem is the obvious censorship on topics like these that search engines now embrace.
The earliest usage of this phrase I could dig up was found in 'darker areas' of the net, in an American right-wing militia newsletter, and used in context to attack the ACLU (link removed from google results, available upon request). It is dated from 1997 and also doesn't give a reference to locate and evaluate the source.
Is that a genuine quote made by Mengele, or is it a made up quote?
Did Josef Mengele really say:
"The more we do to you, the less you seem to believe we are doing it"?