so the mother didn't know??

what do you mean, not knowing whats happening inside the walls? sure she did it was shown multiple times eg. telling the housekeeper she could incinerate her and rudolf calling her about the plans. hedwigs mother found out during the visit.

I think she didn't know what was going on before her visit but put it all together, the noise, the smoking chimneys etc. And imagine the smell. That's why she disappeared in the night, not wanting to even look her daughter in the eye.
IRL who knows who knew what.

Of course she knew, she just couldn't handle the reality of it up close.

Of course she know. All Germans (except the stupid ones) knew what is happening with the Jews! They just didn't WANT to know and that became the generation lie. But they definitely knew that their Jewish neighbours were send away and no one heard of them again. It is hard to believe, that everyone thought they were sent to a happy place. It had worked, the next generation had for sure believed the fairytale, but everyone born before end of WW1 (1918) for sure just didn't want to know.

I found this: https://screenrant.com/zone-of-interest-movie-hedwig-mother-auschwitz-response/

“It’s just the proximity. It’s no different, to someone like her, to buying your steak at Sainsbury’s and going to an abattoir. You know where that steak comes from, but you don’t really want to be around a cow being slaughtered or the smell of it, or have the blood running over your shoes . . . there’s no pang of conscience, no redemption. There’s no salvation in this film, and there can’t be. These characters end the way they start.

“Primo Levi talked about how they were made of the same clay as the bourgeoisie in any country. They really were Mr and Mrs Smith at No 26. They were our neighbours, and our neighbours would say they were us. Those were the basics of what I got from the archival research: how grotesquely familiar and ordinary they were.

“What they were interested in: status, family, health, holidays, possessions are no different to the things most people want . . . The Hösses weren’t born mass-murderers. They were teenagers in love with ideas about the future. That’s how they started. And look where they ended up. There’s a warning in that.”