The other Musketeers know Jacqueline is a woman.
Jacqueline has a Paper-Thin Disguise
: she might get away with it for short periods for people she doesn't interact with much, but up close and personal with the other Musketeers? It makes more sense if Ramon, Siroc, and Duval realize something's up. However, Jacqueline's good at what she does, so they ignore the issue. This explains a few side glances she gets from the other Musketeers; note Ramon's response to her imitation of a female voice in "The Girl from Upper Gaborski," in particular.
Along the same lines...
"Squeaky," the high-voiced Musketeer Duval references in the first episode, was actually a woman.
When Duval reassures Jacqueline that she can be a good soldier despite her high voice, he's actually trying to subtly reassure her that she can be a good soldier despite the fact that she's a woman, and he won't expose her secret.