Your sister is the the most selfish girl I am reading about,and of course her lover also, who maintained relations with her after her marriage,even though he had broken up with her earlier.
Your sister has dug her own grave.Her lover is a very practical person who left her when he got fed up of her nature,restarted relationship with her after her marriage and left her again.I am sure he too realised that if your sister can't remain faithful to her husband,she won't be faithful to him as well,if they ever marry.
In all this drama,her husband has been the biggest sufferer.Why did you marry her off,after keeping her husband in the dark that she had physical relations before marriage?
Your sister is not a baby who needs pampering always,which you seemed to have given her all her life.She wanted things to happen her way all the time,and when it did not happen,she is threatening suicide and a false rape case.
Her false rape case won't stand in court,as there was mutual s*x with her lover.So no forced penetration and injury marks will be seen in medical test.
If you again bow down to her emotional blackmailing tactics and file a false rape case,her smart lover,which he is,will not keep silent.He too will hire a good lawyer who will prove that the s*x was mutual..This will defame your family in society and your sister will be thought of as characterless and a liar,as she had physical relations before marriage,then had extramarital affair,and contd. her relations even after the divorce.
If you have any plans to remarry her,they will also get jeopardised,for she will earn a bad name in society
.
Similarly,don't pay attention to her suicide threats.She is doing so,only to get your cooperation in filing false cases against her lover.Just ignore her.
If she at all attempts suicide,the police will also investigate the matter and her lover who is already fed up of her by now will also retaliate by showing all the proofs of her overpossessive and abusive nature,in form of letters,SMSs,e-mails etc.Your sister must have surely showed her possessive and abusive nature in her e-mails too.Then he may also prove that despite her marriage,she continued her relations with her lover.
If such proofs these are shown to the judge,he too will get convinced that your sister is aggressive,overpossessive and also unfaithful in any relation and has attempted suicide just to gain attention and sympathy.The whole case will go against her and all of you will become a laughing stock and the centre of gossips in your neighbourhood,relatives and so on...Do you want such things to happen?
It's good that you are seeking advice and not falsely siding with your sister.
You become strict with her now instead of bowing down to her all the time.Your sister does't respect any relation and hence has failed twice-once with her lover and once with her husband,where the husband was not at all responsible.
Ypu take her to a good reputed counsellor who will give her behavior therapy and if need be,will refer her to a psychiatrist.
Take the above steps quickly,before it becomes serious.
All the best!