You said it right, even in chain Sita's thoughts were free, even while moving surpi and sulochana couldnt express their thoughts. Surpi still expresses, but sulo she is tied.
@Shruthi & Aditi: this is not just with Sita or Shurpi or Sulochana. This always happens with the women folks.
Shurpi is daughter of the house and even under control, normally she can still open up or flare up to some extent and then doused down or suppressed by parents or siblings if she goes overboard.
Sulochana is the daughter in law and she will have all the possible restrictions like, she should not back answer her husband, express to her in laws, most worst case is like a DIL should not talk openly about her displeased topics in front of the crowd. Even if she is unhappy, she has to pose as a happy woman in front of the society and keep up the charade so that her husband's ego is kept intact. If she back answers or express displeasure openly, the it is considered as a blow to her husbands fat male ego. That is why normally while sending the daughter to her Sasural, the parents will advise the girl to keep her eyes low in front of elders, keep her voice at low decibels and never openly disagree with your husband and reduce his prestige etc.
Sita is an outsider and normally one can't control an outsider from pin pointing their drawbacks as they have no hold their tongue. They have no fear to lose anything even if the person in question is unhappy with their comments. That is why normally it will be an outsider who will be able to show the mirror than the family.
Shurpi will be sent out if she goes against her brother and she has no place to go if he abandons her and out of his fear, no one will even give shelter to her. So it is as good as losing her life which she can't put at stake and hence she could not oppose him even though she wanted to. And Sulochana was a married woman whose life revolves around her husband and she can't go against her FIL as her husband doesn't like it and even she has no place to go as she will not be welcomed back in her maika as well. Then it was considered as once married and gone to sasural, the girl can pay visits to her maternal home only during some festivals or poojas and never welcomed to stay back permanently. So she has left with no opting but to keep mum. That is not the case with Sita. For expressing her voews, tyo the maximum she may be punished to death and she is not worried about death as she feels death is better option than staying away from her husband. So she may choose to provoke Ravan so that he can order his men to kill her than staying in his captivity which is like living in hell.