If Linda and Lenina are the most serious rebels of the New World, why, then, are Bernard and Helmholtz exiled, but not Lenina or Linda?

Without being able to agree with the premise of the question, there is no substantiation that can be provided other than Lenina's somewhat rebellious behavior regarding the "everyone belongs to everyone else" credo.  She prefers having only one boyfriend at a time, such as Bernard, then John; but, she is not above quickly baring herself for someone to whom she is attracted. And, while her identity at the end of the book is not overtly stated, she seems to be the girl sent to seduce John in his exile.  As such, Lenina acts, not rebelliously, but in compliance to Henry Foster, who deserts her to her fate as she is sent by helicopter to where John is exiled.  Lenina is a character who is used; even in the beginning of the novel she says that the men find her "pneumatic"; in Lenina's case it means like a balloon, bouncy,  suggesting the feel of her bosom.  And, since two-thirds of the women are sterilized, there seems little purpose to women in Brave New World.

When she is returned to the New World, about which she has nostalgically told her son, Linda willingly subjects herself to soma and makes no attempts at existential acts, such as John, her son, does.

I really do not see how these two are the most serious rebels.  Their experiences do challenge the assumptions of the dystopia, but it seems to me that this is only because of the way that they are acted upon by the men in their lives.  This shows me that Huxley does not take them seriously -- they are not actors; they are acted upon.

Linda firmly believes in the ideals of the civilization.  She ends up living in a way that goes against everything the society stands for, but it is not anything that she wanted.  Lenina ends up as the love interest of John.  The idea that someone could be the object of love is anathema to the society as well.  But this is not Lenina's doing, it is John's.

I think that Huxley does not take the women seriously because he just has them be objects of what the men do, for the most part.  But the fact that he draws them in this way makes me unable to see them as true rebels.

First, Huxley definitely does not take his main female characters, Linda and Lenina, as seriously as he does the men. The main male characters are alpha-pluses or, in the case of John the Savage, have the intelligence of an alpha-plus. Linda and Lenina are betas, and this shows in how they are treated in the narrative. They never get together to discuss ideas, as Mond, John, and Bernard do; further, their lives are depicted as inane, centering around materialism, sex, soma, and sociality.

On the other hand, both characters do reveal some complexity. Linda, for instance, shows a great deal of agency when she manages to survive after being abandoned and left for dead on the Savage Reservation. Although what happens to her—pregnancy and childbirth—are the most shameful and degrading things that can befall a woman in her society, she rises to the occasion. Despite having been taught it is sick and neurotic to establish a family unit, she cares for and nurtures John, even teaching him to read. Further, although her social mores are completely opposite of those of the reservation, she somehow manages to make a life for herself. She is not, however, a rebel, because she dreams of returning to the World State and resuming her own life. When she is able to return, she spends the rest of her life in a soma haze.

Lenina is actually more rebellious than Linda, though she doesn't have to cope with the kind of crisis that faced Linda on the Savage Reservation.

As the novel opens, Lenina's friend Fanny is chiding her for dating Henry Foster exclusively for four months:

It’s such horribly bad form to go on and on like this with one man.

However, Lenina is enjoying monogamy:

“Somehow,” she mused, “I hadn’t been feeling very keen on promiscuity lately. There are times when one doesn’t."

While Fanny will simply refuse to limit herself to one man no matter whether she wants to, Lenina shows her rebellious streak by stubbornly staying with Henry for four months.

Lenina also shows a rebellious streak in the dangerous and different men like Marx and John who attract her. While she doesn't understand either of them, she also doesn't want to be with someone boring and ordinary. She is curious to visit the Savage Reservation, and she reaches out to John perhaps past the point she should, given how he treats her. It's as if she wants to be with a man who will broaden her horizons but can't manage to overcome her conditioning enough to receive what they have to offer her, yet her rebellion shows in her reaching out to them.

Lenina challenges the dystopia of Brave New World by proving that even a woman who has been conditioned to hate babies and families and motherhood can actually develop a motherly bond to an infant she has given live birth to. While she is on the Savage Reservation she gives birth to John, and while she struggles with motherhood, she does (in her own awkward way) find ways to nuture him through teaching him to read and finding him some father figures (pope) to help him navigate the world.

Lenina rebels against one of the main tenants of BNW (Community). While she has been raised to believe that everyone belongs to everyone, she is led by John to explore the meaning of true love by studying Shakespeare. She learns the meaning of romance and falls in love with John by the end.

Huxley takes these women perhaps the most seriously, as they were the ones who disproved some of the most sacredly held beliefs of their culture .

In some ways, Linda and Lenina are the most serious rebels of the brave new world. How does the experience of each character challenge the assumptions of the dystopia? Do you think Huxley takes these women characters as seriously as he does the men? Why or why not?

There isn't enough space allotted to get into a decent answer for the first part of your question about how the experiences of Linda and Lenina (note the similar names) challenge the assumptions of the dystopia.  However, the eNotes character pages given below are great reference points to get you started successfully down that path.

As for the other, more subjective questions, I believe you could argue it either way.  You'd want to put parameters on "seriously", though.  As a writer, he takes them seriously enough to include them and make them relevant to the men whose lives they are a part of, but does that men that Huxley himself, personally, took them seriously -- that's only answerable by Huxley.

Personally, I do believe that these characters are as important as the men because they help to bring certain understandings of the two worlds into the reading that otherwise would not be obtained.  As a woman, though, it may be, admittedly, that I am identifying more to them than to Bernard or John, their counterparts.  Huxley does seem to put a great deal of energy into developing these characters, more so than Fanny, for example.  She's really only a part of the story to help us understand Lenina.  Though, one could argue that Lenina and Linda are only there to help us understand Bernard and John.

What do you think?  Did you feel like Huxley was taking these characters as seriously as the men when you were reading the novel?

It's a great question; good luck with it!