All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the documents that prove the first marriage are fake, then the second marriage of your sister is valid. So, you may visit her, and it is recommended since it is an act of keeping ties of kinship. If the documents are genuine then the second marriage is null. Therefore, she must separate from the second man since the relation with him is no longer valid and it is considered pure Zina (fornication or adultery). A married woman is not allowed to marry someone else. Allah Says when speaking about the women who are prohibited for us for marriage (what means): {And [also prohibited to you are all] married women…} [Quran 4:24]
If your sister commits the second marriage contract after having the knowledge of prohibition, then she deserves severe punishment i.e. stoning till death. The man also deserves the same punishment since he was already married. Know that only the Islamic court can enforce such punishment.
However, if the Islamic rulings are not carried out, then at least they should receive the punishment that can deter them from such violation of the Sharee'ah rulings. Those who can separate them should be informed to remove the evil.
The ruling of visiting your sister depends on the purpose of this visit. If you intend from visiting her to exhort her, to advice her to repent to Allah and leave this man, then it is recommended; yet, it becomes compulsory if it is the only way to stop her from this evil.
If there is no such benefit from visiting her, then, at least, you should desert her after spending all efforts to separate them.
Allah knows best.