Sister tourcher by husband , father in law . mother in law

Hi , My sister married a year ago and after 3 months her in laws and husband blamed that she is characterless as she is chatting with another guy by what's app and do not like her husband. They have stripped her phones and jewelry and all the cash and smartly send her to our home. However that is not the case they want dowry and insult to my sister as they have asked to us and come for marriage . They have provided the fake hard copies of the messages and fake address with date of birth to some mediators just to insult. And now all the socially peoples are trying to prove that and they are giving different different reasons. please advice how we can take the evidence as mediators are trying to send my sister respectfully in the meeting but he is denying to have any signatures by in laws on agreement, which will proof if in future any instance or any misshaping will happen then she would not responsible and her inlaws will be responsible as she has not done any thing wrong earlier. Kindly suggest what needs to be done in this meeting and before send her in laws.