You've said "I'm the aunt" out loud dozens of times.
Then a stranger at the grocery store assumes again.
One donor asked a simple question online. What should she say if the kids ask?
Thousands of upvotes told her to stay in her lane.
That thread shows what your one sentence has to do.
The Sentence You Keep Saying Only Fixes the Moment
You have said "I'm the aunt" more times than you can count.
It clears up the moment.
It doesn't reach the next stranger.
Repetition works on one listener at a time.
So the firm tone you've perfected has a ceiling.
A better fix has to work before anyone asks.
She Only Asked What to Say
That donor wanted to know what to say if the kids asked her directly.
That was the whole question.
A comment telling her to stay in her lane collected thousands of upvotes.
Nobody answered what she asked.
They answered a different question, about what she was.
It's a strange thing to be told about yourself by people who've never met your family.
The Crowd Had a Point
Family privacy is real.
A child's origin story belongs first to the parents raising that child.
Your sister called the two-week wait "the longest, worst two weeks ever."
She earned the right to tell this story her way.
A donor who pushes can look like she wants a vote she never earned.
Fair. But the crowd stopped one step too early.
The Other Side Had a Point Too
Others in that thread said children have a right to know their origins and medical history.
Medical history follows a body around.
A doctor's questionnaire someday won't ask what the family agreed at the table.
You share about 50% of that child's genes.
Both camps are defending something real.
Neither one was standing where you stand.
Neither Side Asked the Person in the Middle
The crowd argued about a principle.
You have a toddler niece and a monthly visit on the calendar.
You are the one who will be at the table.
You are neither parent nor bystander.
That position is uncomfortable, and it's also useful.
It lets you see the parents' fear and the child's future at once.
The Label Was Never Only Yours
You and your sister agreed on "aunt."
A lawyer's agreement covered parentage. It didn't cover what to call anyone at Thanksgiving.
And the child hasn't weighed in yet.
One donor-conceived commenter pushed back that some donor-conceived people "do think of the donor as our mother."
You can be a loving aunt all the way down.
The child may still choose her own word for you.
A Secret Is a Story Someone Else Will Tell
A plan like "tell her at 18" sounds tidy.
Donors and commenters warned it turns the story into a betrayal.
The fear is a child hearing that "everyone has been keeping a huge secret from you your entire life."
Compare that with a story told simply from the start, through books, with no dramatic reveal.
Nothing about that version takes anything from your sister.
It takes away the ambush.
The Good Part Gets Lost in the Argument
Both camps skip something.
One donor put it this way: "The joy on my sister and her husband's faces when she was born was priceless."
In another account, the trip brought the sisters closer.
That's the good an honest story protects.
A child can grow up hearing it as a generous gift from an aunt.
That brings us back to the sentence you wanted.
The Sentence That Actually Does the Job
It has to respect the child, your sister and you.
Try this: "I'm her aunt, and her mom is my sister."
It answers the stranger in one breath.
It hands over nothing about how the child came to be.
That part stays the family's story, told by the family.
Say it in a level voice, with no apology, and it still sounds like you.
You Might Be Overcomplicating It
Maybe it's already fine.
You said aunt, everyone agreed, and the child is loved.
That's a real win.
Yet a label that lives only in the adults' heads gets tested at a holiday.
Sisters in one account grew closer on the trip.
Months later they fought over how to describe the donation to the child.
The love was fine. The script was missing.
Asking the Question Doesn't Make You a Threat
Speaking up feels like overstepping, especially after a crowd tells you to stay in your lane.
You can ask your sister to settle the script without asking for a vote.
Try "What do we say when someone asks?" and let her answer first.
Some donors also feel too many feelings around the child and step back for a few weeks.
Say so early, so a pause reads as care.
Say it before it can be mistaken for withdrawal.
Whose Story It Is
The child's story belongs to the child.
The parents hold it first, and carefully.
You hold a piece of it, and a piece of the child's medical history.
Holding that piece well means you don't hide it and you don't seize it.
One donor says the child "truly just feels like my niece."
Call your sister this week, before Thanksgiving does the asking for you.