In previous scenes Ripley is shown to be mentally and physically depleted, not least since she has been told (by Burke) that her daughter (and every other personal relationship)is gone due to her extended hyper-sleep. On top of this her intelligence and professional ability has been demolished - she is demoted after being accused of acting irrationally in destroying her ship (the suggestion is she may be culpable for the crews deaths). She is shown to struggle with the request to go back to LV 426 initially rejecting it. But what else does she have? Certainly no purpose in her life at that point and in a steep decline.
So, by the time she gets to the ship and the scene you mention, she is already fatalistic, suspicious of officials and unsure how much everyone else in the group knows - in this situation you wouldn't divulge sensitive information to an unknown group of people you have just met concerning the motives of the corporation (Weyland) that essentially owns the environment you live in and the mission you are engaged in. You would keep quiet and let everyone else talk - the same as you do in a corporate, military or social situation if you have any sense. Think before you speak or act.
Once Ripley has evaluated the situation and found the team to be naive and wholly wanting (after they get there 'asses kicked' under the reactor)- she steps up and asserts herself, taking command in all but rank.
Short answer - Ripley has nothing to gain at this point in the mission by disclosure (about previous events, the corporation's real motives and lack of concern for the crew / colonists) - she did this at the board review already and it fell on deaf ears. She has now determined her own purpose and mission now - to go to LV 426 to destroy the alien investation.