People do mistakes, but one should not do mistakes so grave that one cannot correct especially in marital life, this fact your sister might have understood by now.
Going for 21b application in HC, asking for speedy trial will further prolong the course of justice.
Actually, in matrimonial matters there is no justice served. If he has taken dowry, he will be punished in accordance with law. But in the first instance if he has taken dowry, why did your sister give it? Dont you know it is a punishable crime in eyes of law? If her husband wants can also file a case against the whole lot of you, for giving dowry. You are as much in vulnerable position to go to jail as he is.
Contrarily if you fail to prove that he took dowry, he can file perjury case on your whole lot, and lying under oath is a punishable crime upto 7 years jail for it.
What happens usually in cases like your sister's is that, trying to prove a point will prove costly.
You guys got concerned, having one girl at home was one problem now there are two, let her go back, if this is true you should have thought very deeplly before filing 498a. By doing that you guys closed all doors for reconciliation.
People such like your brother in law, when do marriage, they do it on basis of trust, rather than photos, inviting people, grand function. To prove bigamy some argue that wedding card is enough, photos are enough, court itself will come out with middle solution for the problem if anybody has to go to jail. Make note of this point.
Your BIL has made up his mind, he will face what come his way in the future.
I feel sad about your sister, why give dowry and then complain, why not adjust with husband and his folks, the girl now with your BIL isnt she adjusting, I bet they are not married and are just in trust livein relationship. Proving all these facts also will take lot of years.
Finally where is justice? Dont you think your sister did some wrong somewhere? You might not agree like scores of other family members and brothers who have filed such cases, but who is suffering?
Your sister did wrong by rejecting settlement money then, I suggest your sister to go for settlement even now, her husband has moved on, she should also move on, by taking mutual divorce and hefty one time alimony if her husband is still willing to give.