All Perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as that of his family and all his companions.
If the reality is (as you mentioned) that the man wants to conceal his marriage and does not want to make it public, then we advise the questioner to keep away from this matter and not to help this man. Muslim jurists called this type of marriage “secret marriage” and they disagreed in its validity. Some are of the opinion that it is valid provided it meets all the conditions of an Islamicly valid marriage; most important of which is the presence (or consent) of the guardian of the bride. See the details in Fatwa 81385.
Although some ruled it as valid, yet, according to them, it is not desirable. On the other hand, other jurists are of the opinion that it is totally void, especially if both parties agree on its secrecy.
Therefore, you see that this marriage is a matter of disagreement among jurists and at least it is undesirable according to those who permitted it. The reason for it being undesirable is that it contradicts to the order of the Prophet of announcing the marriage. Additionally, there are many other disadvantages, like the lose of rights (of the wife) that may result from such a marriage. You are not obliged Islamicly to help this person, so you should keep away from him.
Moreover, the wife's sister is not Mahram (i.e. a spouse or a person whom one is permanently prohibited from marrying due to suckling, marital, or blood relations) for you as we mentioned in previous Fatwa 84623 and also please refer to Fatwa 84143 to know the definition of a Mahram.
Allaah Knows best.