Okay, first of all, unlike many in the Mario and Luigi fandom, I actually believe Cackletta made it to the afterlife. It is clear, even more so in the remake, that the remnants of Cackletta's Soul (her head basically), gets carried away to some type of dark abyss. She gets sucked away into a portal/vortex of some sort not unlike the way spaceships tend to beam people up into them. It is obvious to me that she gets carried away somewhere. If she were truly erased from existence, I don't even think her head would've made it out of Bowser, and certainly wouldn't look like it was getting carried away somewhere.

As for Antasma, I think when he blew up, his soul remnants just popped up in the afterlife.

Cackletta spends six years alone in the Underwhere as only a head in punishment of her pride and vanity. When Fawful ends up blowing himself up and lands in the Underwhere, he tells Cackletta off before leaving her for good, which devastates her, and she cries over him day and night, becoming known as "The Witch of Everlasting Sorrow". When Antasma ends up there four years later, he falls head over heels for her, thinking she is so beautiful, while also feeling pity for her over her mourning. His affectionate way of treating her grabbed at her tender, vulnerable heart, and she sorta fell for him as well. They were all that each other had, and spent all of their time together, being lights in a dark place for one another. They actually promised each other that they would get married if they ever came back to life.

Now as for HOW they came back to life, in Cackletta's case, Queen Bean ends up missing her so much that she calls on Princess Peach to help her revive Cackletta with the Beanstar. Cackletta ends up awaking in Dimble Woods in the Mushroom Kingdom, where she currently dwells until post-Of Love and Revenge. With Antasma, I really don't know for certain how he would come back. I will say that in one of my stories he actually gets revived with Cackletta at the same time since he is clinging to her when the wish is made.

They both live together deep within Dimble Woods in an abandoned Toad House. Cackletta, being a witch, is well acquainted with herbalism and begins to make herbal medicine, as well as soaps, perfumes, lotions, etc. to sell at the Toad Town farmers market three days a week. Antasma primarily forages for some of the things she needs for what she makes. It is a relatively hard life for the two, but surprisingly enjoyable. Cackletta of course, transforms into a Toad prior to going to the farmers market.

Not long after they come back to life, the couple get married, with Toadette being the one to marry them.

The couple become ever closer, with a very healthy emotional and intimate relationship. They don't "deprive" one another, if you get me here. While they are both adamant on getting revenge on the plumbers that killed them, their love for each other is largely what delayed their plans for ten years. That, and having children.

Cackletta is an old lady approximately around 60ish. So yeah, she is past childbearing age. Antasma especially was desperately wanting kids, but she had to break the bad news of her inability to have children, which put a bit of a strain on the two. Since Antasma made a wish to the stars to maintain his battle form at all times, and that wish came true due to him doing it for Cackletta's sake, Cackletta decided to make a wish to the stars to conceive and carry a child for Antasma's sake. It came true! The couple were overjoyed.

Childbirth for Twilheena was rough, taking a little over a day. When she was born, Cackletta felt her heart swell with love for this little girl, and she cried tears of joy. Something changed with Cackletta after having Twilheena, as she became a bit softer and more gentle than she was before, and she always treats Twilheena rather tenderly. Unfortunately, a few days after childbirth, Cackletta suffered horrible postpartum depression that felt akin mentally to how she felt when her soul exploded.

Of course, Cackletta wished for Heealuna as well, but the childbirth took over two days and the enduring pain and struggle to have Heealuna resulted in Cackletta dying again. Luckily, a 1up mushroom was around. She treats Heealuna softly as well, although she has to get firmer with her due to the kid's troublemaking ways. Heealuna ended up getting closer to Antasma when she was younger, just like Twilheena was closer to Cackletta when she was younger.

Cackletta and Antasma ended up gaining the loyalty of several bats around the Mushroom Kingdom and beyond, with the most notable ones being a quad (quint?) of bats especially loyal to Cackletta named Nosefourrahtwo. With that being said, they have several other subjects as well, truly becoming bat royalty.