What are your daughter's interests? And/or natural talents?
She does not have to do what brother is doing.
For me, per my kids, they do what interests them and what they have a passion for.
Interests can also change.
Per age. Per stages.
When a kid likes what their interests are, there is less forcing it.
Maybe your daughter doesn't want to quit, because she can tell it is a family thing... that she do something. Or she doesn't want to seem like a "failure" in comparison to everyone in her family.
Or to be teased, by others on the team.
And maybe, she is afraid to say, what she really feels or thinks, or what is in her heart. Because... she is compared or feels she is.
And some kids burn out. Already. At a young age from a certain sport.
Her brother is a "natural athlete who loves swimming..." but she is not.
And that is okay.
She needs to know... that.
What is SHE a "natural" at?
Or what does she love to do?
Not everyone is an athlete.
Not everyone likes to play sports.
And not everyone has to.
But they can do it on a recreational basis, or via a formal team/organization or for fun.
My kids like to do tennis, bike, martial arts, art, and music So that is what they do.
But not both kids are the same.
Your daughter is already, at 11, "comparing" herself to her brother and others... and knows... she is not "as in good shape... and needs to condition her body."
That is a loaded feeling, for a girl, a Tween, who is 11 years old.
Body image concerns already.
Hmm.
When I was a kid, my parents, or more like my Mom, made me take piano. I HATED it. I did it though, because we were supposed to. But on my piano lesson days, I would actually runaway from home. After a year or so, she finally let me quit. All along I was told taking it was good for me, blah blah blah. So what. I didn't like it.
But I liked... to play the Oboe. And later, I was a track athlete. All on my own... volition and choice. And then, I really did excel at it. I was in the top of the team.
My sister on the other hand, was NOT an athlete type. So fine. But on her own, she did do fitness things or at a gym. Fine.
Everyone is different.
Also, at different age stages, the body of a girl, changes. They grow both horizontally then in height, then horizontally then in height etc.
Their body shape is not static. And after being in swimming for now, on this second year... she is still working on "conditioning" and is still not as in good shape?
Did her Pediatrician say, she is overweight? Or that her growth/height/BMI proportions is a concern?
If not, then I would explain to her that she does not have to fret over it.
A girl's body-image, as a Tween and Teen... for some girls, are really delicate.
Ideally, she can do something that she likes. And is unique to her.