Their intention is that you react to the news. The outcome that they expect is that you agree for mutual.
As you have heard the saying "Barking Dog seldom bites", if they intend to get him married secretly, then why would the husband's brother call and say? He could have just asked your sister to agree for mutual divorce, instead of giving the reason of 2nd marriage. So, you got to think strategically.
On the other hand, as pointed out by you, your sister cannot do much if at all they get him married.
Sometimes you got to allow them to make mistake. If they make this mistake of getting him remarried, then they are in deep s**t.
Also, when second marriage (when first marriage is in existance) is itself illegal, the question of not taking action (by your sister) does not arise.
You may get responses from other advocates to file a police complaint etc. But I suggest, you to just wait and do not take any hasty decision and allow them to make the mistake.
BTW FYI the court will ask proof even if you say they are planning to get him married. See, they are not going to give you photographs etc. for you to show it to the court. So there is no way you can prove it also. Even if they are sleeping together, you cannot prove it that they have been married. The only time that you can prove is when the child is born and father's name is that of your sister's husband.
If at all you go to Police, they will ask the same thing as to who informed you. At the max they will go there and enquire. And suppose you do this some 2 to 3 times, then the Police will stop cooperating with you and will ignore you, which is what perhaps your sister's husband wants, so that finally after 3 or 4 times, they can get married.
What you can perhaps do is negotiate a higher alimony if they are in a hurry.
Give me a call and I can explain it better.