Divorce by mutual consent is possible if both parties are available, if one of the party is untraceable, how that will be done. If your brother is untraceable, you are left with no other option but what I suggested you, the family court in the divorce by mutual consent want the presence of both parties for dissolving marriage by decree of divorce.
Other than this is someone suggest you for bring your brother's ghost for divorce by mutual consent than I'm sorry I can't say anything more for this.
As far how to proceed with the case as I suggested you, firstly engage the lawyer who has mindset to understand the situation because many jokers are available everywhere to suggest all the things that will simply confuse you all and lead you nowhere.
Secondly the divorce case against your brother has to be filled by your sister in law and follow up the complete process as I mentioned above. You just tell all this to your sister in law to go ahead with divorce case rather waiting for your brother to return for divorce by mutual consent.