The currently-accepted belief:
Currently, it seems as though eating Mother Carmel allowed Big Mom to inherit the power of the Soul Soul fruit (aka, eat a devil fruit user to gain a devil fruit power). However, logically this can’t apply, as the fruit will reincarnate on a nearby tree or nearby similar fruit upon the user’s death. Being eaten alive would, expectedly, result in death.
But what if Mother Carmel didn’t die?
Consider the following:
After meeting up with Streusen, Linlin became a pirate at age six, gaining a 500,000,000 berry bounty by age 10. She mothered dozens upon dozens of children to accumulate more power, just like Mother Carmel. To her own children, she is as kind and uncaring beyond their inherent use to her, just like Mother Carmel’s true intentions towards the orphans.
After falling into the ocean after King attacked the Queen Mama Chanter ship, she (somehow) washed up on shore with no memory of her identity. Her personality is kind, and generous, happy, and even gullible – the exact same personality we saw in her flashback in Elbaf. And as we know, falling into the ocean weakens Devil Fruit users, and in the case of Moria’s shadow-infused zombies, sea salt would nullify the power of his fruit.
When Linlin began eating everything in sight at her birthday, at some point she grabbed Mother Carmel. She knew that Linlin’s strength was immense, so there was no reasonable means of escaping or telling her to stop eating.
At that point, I posit that Carmel’s instinct to survive kicked in. We know that the Soul Soul Fruit can infuse soul bits into inanimate objects. An awakened Devil Fruit extends the ability beyond its normal limits, so I theorize that Carmel is an awakened user, and that an Awakened Soul-Soul fruit can infuse souls with other souls, and that she did this right before her body was mutilated from being eaten.
In this way, Carmel survived, but only to live a half-life. While she is technically alive, she cannot control Linlin, but influences Linlin’s own personality to adapt her own personality. As a result, LinLin becomes a rotten, cruel human being obsessed with accumulating power, raising a family of her own for the purpose of building an empire, under the perceived guise of Linlin’s original goal of building a utopia where everyone from all walks of life can live together in peace, but at the same time, is willing to be cruel to meet those ends, and gains Carmel’s powers, because their souls are fused together as one person via the active power of the Soul Soul fruit.
Falling into the ocean negated Carmel/Linlin’s powers, which is why Napoleon becomes shy. The negation of powers seems to be dependent on distance from the user, though, as Zeus was doing just fine helping Nami attack the bath house at the time (at least this seems to be at the same time but I could be wrong). But during this time, O-Lin is the same kind human we met in the Elbaf flashback, and gets mad at Luffy only for not sharing food.
The head trauma from Queen’s attack “woke up” Carmel and the active devil fruit power of their fused soul, restoring Big Mom’s memories, and her awful personality immediately returns before she crashes for an impromptu nap.
So there you have it. I suspect Big Mom is technically an amalgamation of Charlotte Linlin and Mother Carmel. There’s also the fact that Big Mom named all her children after foods, just like Mother Carmel’s name, but that’s too weak to stand here (it could just be a thing she does to honor Mother Carmel if this theory isn’t true).
*Theory by grizzzlay85