Boromir and Faramir work for me really well in this regard, too, and I agree that "older, protective" might make the difference to how Fritz in rl dealt with AW. But I still felt this made the fic's version of Wilhelmine into practically an OC (and her bond with Fritz into a new relationship), because someone without any damage, her own emotional needs and mistakes are not a version of Wilhelmine I still recognise as her, which is my criteria for AUs. I mean, I know she's just a supporting character and mainly there so that AU Fritz has someone in his childhood. And it's not that I wish miseries on any version of Wilhelmine! But I do think her share of trauma made her, well, her, instead of marvelously normal and supportive female character X.

To go back to Boromir and Faramir again, they're a case of protective older sibling who is not resented by abused younger sibling despite being favored by their father, but Boromir, both film and book version, is anything but idealized. It's hard to compare, because as opposed to AU Wilhelmine he's a main character in the first part of the book/first film, and thus gets way more narrative space to be fleshed out. But let's assume a Gondor-centric version of the story that starts with Faramir's dream and stays with him. Boromir would appear a sympathetic character in regards to Faramir, but there would, presumably, still be some indication on how their father's beliefs and attitudes have formed him as well.