In Fantastic Beasts: The Crimes of Grindelwald we have a flashback scene where Leta is travelling on a ship to the US with Irma and her baby brother, Corvus. Across the cabin from them we also have Credence travelling with a woman. I saw no reference to who this woman was in the film.

Who is she?

Credence's Aunt

She is solely mentioned to be Credence's aunt in the screenplay.

SCENE 109
EXT. LIFEBOAT—1901—NIGHT

CHILD LETA, IRMA, and BABY CREDENCE are in one boat, CREDENCE’S AUNT and BABY CORVUS in another.

_{Fantastic Beats: The Crimes of Grindelwald - The Official Screenplay}

Considering that Credence is said to be Albus' brother it is highly likely she is also Dumbledore's aunt. This is of course assuming that Grindelwald is telling the truth which may not be the case.

SCENE 120
INT. NURMENGARD CASTLE—DAWN

GRINDELWALD

You have suffered the most heinous of betrayals, most purposely bestowed upon you by your own blood. Your own flesh and blood. And just as he has celebrated your torment, your brother seeks to destroy you.

[...]

GRINDELWALD

It is your birthright, my boy. As is the name I now restore to you.
(whispers)

Aurelius. Aurelius Dumbledore.

_{Fantastic Beats: The Crimes of Grindelwald - The Official Screenplay}

Of course the Dumbledore known family tree isn't exactly extensive but we do know of an aunt of Albus' called Honoria so it could be her (quote taken from Pottermore).

Though somewhat dated, the expression ‘to have a hairy heart’ has passed into everyday wizarding language to describe a cold or unfeeling witch or wizard. My maiden aunt, Honoria, always alleged that she called off her engagement to a wizard in the Improper Use of Magic Office because she discovered in time that ‘he had a hairy heart’. (It was rumoured, however, that she actually discovered him in the act of fondling some Horklumps, which she found deeply shocking.)

_{The Tales of Beedle the Bard, Chapter 7, "Albus Dumbledore on 'The Warlock’s Hairy Heart'"}

The screenplay says she’s Credence’s aunt - no more is said.

In the Fantastic Beasts: The Crimes of Grindelwald screenplay, the woman with baby Credence is refered to as his aunt but not named, and described as a dark-haired young woman.

The cabin door bangs open to reveal a dark-haired young woman wearing a nightdress and life jacket.

CREDENCE’S AUNT

Irma? They want us to put on life jackets!
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

No more than that is known or mentioned about her as of yet, which is almost certainly intentional vagueness. Since it’s still unclear who exactly Credence really is, it’s unlikely they’d want to make it clear who his aunt is, since that could be a clue to his own heritage. Though Grindelwald has told Credence that he’s really Aurelius Dumbledore, this may or may not actually be true.

Given room at last, the bird stretches its wings and becomes full size. The bird is aflame, a phoenix reborn.


GRINDELWALD

It is your birthright, my boy. As is the name I now restore to you.
(whispers)

Aurelius. Aurelius Dumbledore.

(beat)

We will go down in history together, as we remake this world.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

It’s possible that Grindelwald is telling the truth, but it’s also possible that he’s lying to manipulate Credence into both feeling grateful to him for showing him his heritage and hating his “brother” so he’d be more willing to kill him as Grindelwald is planning that he would.

GRINDELWALD

Who represents the greatest threat to our cause?

KRALL

Albus Dumbledore.

GRINDELWALD

If I asked you now to go to the school where he is hiding and kill him for me, would you do it for me, Krall?

(smiles)

Credence is the only entity alive . . . who can kill him.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Grindelwald sees Credence as the key to his plan to kill Dumbledore, so he’d have a definite motivation to lie. Furthermore, knowing that Credence yearns for love and family, he clearly sets a path for Credence to follow and learn his heritage from him, killing off anyone else who’d know it.

GRINDELWALD returns his gaze to the vision of CREDENCE suspended in the center of the drawing room.

GRINDELWALD

The path has been laid, and he is following it. The trail that will lead him to me, and the strange and glorious truth of who he is.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

What he says likely isn’t meant to be taken as truth quite yet, and if it is actually true, that’ll be made clear in the later movies. Who Credence’s aunt is would of course depend on who Credence himself is, which is a question that still as of yet remains unanswered. It’ll likely be made clear, or at least clearer, who she is when future movies reveal Credence’s true backstory.