Not remembering this twist, I did some research. The original being German, I turned to the German Wikipedia page for Hänsel und Gretel. Here is what I came up with:
In the original edition of Die Kinder- und Hausmärchen (1812) the mother had died when the two come home (directly from the witch's house) without any reference to a cause for her death. (People died a lot back then, so I suppose none was needed.) The 4th edition (1840) changed "mother" to "stepmother" (an evil stepmother being a common theme in fairy tales). For a 5th edition (1843), the tale was changed to make the stepmother and the witch appear more similarly wicked (by putting similar or even identical phrases into their mouths).
Ludwig Bechstein in his Deutsches Märchenbuch (1856) closely followed the second edition, although he added a few twists. One of those is that the mother didn't die, but regrets bitterly (until the children show up with all the gems of the witch, ending their misery).
The chapter about the sources of the tale lists many influences, namely similar fairy tales of German and other (European) origin. Those used to be purely oral traditions up until the Grimms' times, so there were many national and local variations (and family-private inventions) on similar plots. Notably, Wikipedia mentions that Wilhelm Grimm was influenced by August Stöbers' Das Eierkuchenhäuslein (The Pancake House, published 1842) that was itself based on the Grimm's original version of Hänsel und Gretel, thereby further obscuring what could be considered "the original plot".
Given how freely the Grimms modified their version of the story between editions, interweaving oral versions of unannotated origins with different literal sources, it wouldn't surprise me if later (post-mortem) editions made further changes, even substantial ones.
I remember being aware as a kid that most of the fairy tales come in different flavors, with extended or cut plots and with subtly or obviously added or removed twists. Most notably, the language was modernized. (Much of the original vocabulary does not make much sense to German adults nowadays, let alone children. However, simply adapting words sometimes breaks rhymes, preservation of which lead to notable changes in some places.) But many later transcriptions also considered the final fate of the evil characters overly cruel and allowed them to meet a milder end than in the original stories, introducing yet more changes.
And translations (one of which is what you seem to be coming from) were probably even more prone to changes. Most of the original translations are based on the 1819 2nd edition, which substantially extended many of the classic tales based on feedback the brothers got from the first edition, but does not contain the changes I listed above.
Baseline: Originally it was just a mother in dire poverty which, threatened to be starving, talks her husband into abandoning the children in the woods, and had to die off-stage before the children come home (probably to avoid the confrontation). Later editions and adaptions either turned her into an evil stepmother or made her regret bitterly what the couple had done to the kids.
(BTW, when judging that mother you need to be aware of the much, much harder times back when these tales were spun by the people. Infanticide was much more common back then. I remember having read somewhere that the church lobbied for not letting babies sleep in their parents' beds anymore, because so many of them mysteriously died in hard times. Parents were more likely to spread their genes if they relied on their ability to make new babies if their kids should die, rather than dying themselves instead. Back then orphans had next to no chance to spread their parents' genes.)
As a sidenote: All of my great-grandparents were born less than a century after the original edition of Die Kinder- und Hausmärchen was published (now 200 years ago), so they might have been read this tale from the first edition, or any of the later ones, or from Bechstein's variant, or they might even have been told any of the orally passed versions by their great-grandparents – thereby making all of them more or less direct influences for my perception of the story. Add to that the modernized versions I read later,
Before this research, if I would have had to tell the story without a book at hand, I would have told Bechstein's ending (mother regrets), firmly believing this is what the Grimms originally wrote. That makes me wonder which version my kids consider canonical...