I took it to mean Fritz' behavior towards his siblings in general, and the critical depiction of Fritz as King (not of his decisions as monarch, of his behavior) in the memoirs. Don't forget, if you simply go by the Wilhelmine/Fritz correspondance and nothing else, then, their own big crisis aside, you've just sentences like when Wilhelmine's replies to his "let's commit suicide together" letter where in the cause of lifting his spirits she says among other things that all his family loves him and adores him, which, well, not so much. Also, after their reconciliation in the summer of 1746, Wilhelmine and Fritz go back to the old intense and affectionate tone, whereas her corresponding letters to AW show she felt scarred by the whole thing and it took her a while longer to go back to trusting Fritz completely again, which can be seen by the fact that when she finally talked about Marwitz (female) having that affair with the Margrave and basically blackmailing Wilhelmine on the terms of "if you don't make your brother give me my inheritance, I'm never leaving Bayreuth and will continue my affair with your husband", she confessed this to AW first and wanted his advice on whether or not to come clean to Fritz. (He did advise her to go ahead and tell Fritz, which turned out to be the correct thing to do.) This was after their reconciliation - in 1747 or 1748, even, I'm not sure which one - and that she asked her kid brother and was still insecure whether or not Fritz would help her or explode at her again is telling about the longer after effect.
(I think it's also telling that very late in the memoirs when she's describing the visit by Fritz & Co. en route to Straßburg, she after stating her amazement how AW has grown up etc. since last she saw him says that just so there is no confusion, from this point on "my brother" means AW, and "the King" is Fritz (who previously was "my brother" throughout). You bet that sentence was written at the height of the quarrel.)
Now, Ziebura says "modified", not "changed utterly", and by the 1750s, Wilhelmine did feel comfortable in her standing with Fritz again and during AW's last year of life was the one trying to achieve reconciliation etc., and Ziebura does point this out (also that Wilhelmine was the only sister who actually dared to plead AW's case to Fritz). But in terms of the big Firstborns crisis, the AW-Wilhelmine letters certainly do modify the picture solely gained by the Fritz-Wilhelmine ones.
Re: AW readthrough
(I think it's also telling that very late in the memoirs when she's describing the visit by Fritz & Co. en route to Straßburg, she after stating her amazement how AW has grown up etc. since last she saw him says that just so there is no confusion, from this point on "my brother" means AW, and "the King" is Fritz (who previously was "my brother" throughout). You bet that sentence was written at the height of the quarrel.)
Now, Ziebura says "modified", not "changed utterly", and by the 1750s, Wilhelmine did feel comfortable in her standing with Fritz again and during AW's last year of life was the one trying to achieve reconciliation etc., and Ziebura does point this out (also that Wilhelmine was the only sister who actually dared to plead AW's case to Fritz). But in terms of the big Firstborns crisis, the AW-Wilhelmine letters certainly do modify the picture solely gained by the Fritz-Wilhelmine ones.