I have studied medicine, hence I prefer to reply to your query.
HBSag positive. This is a form of hepaitis. Hepatitis is inflammation of the liver. Causes of this can be many. She might have contracted this through blood transfusion or some jaundice infection when she was a child or a kid in the early years. This will stay positive for the rest of life of the patient. Per se no cure for this. There is one drug to treat this, but that will only subide it further and not eradicate it. It will stay in dormant position entire life, provided dietary restirctoins are taken seriously and followed as advised by the Gastroenterologist. There is nothing to worry now, death is certain, complications if any will take a long long time to appear (during fag end of life).
This is not a reason to seek divorce. Hindu Law and Hindu Marriage Act does not recognize this as a ground for divorce.
IF both have HBSag positive, let them live together.
What do you want? TO tie him off to some other girl and spoil her life? Anyway they are married. Let them face the problem together, at least this will keep them together.
And for your kind information though it can be trasmitted s*xually you cant say it was due to she having s*xual relations as there are other factors involved.
Putting this kind of information in general public/forum risks attracting case of sharing confidential information which is against privacy of patient. You might be prosecuted for it.
Even if you approach court of law with your above arguments, be ready to face perjury and do time in jail 7 years for failing to prove allegatoins made against woman and sanctity of woman in court of Law.
Your case will fail on the following grounds,
The disease is not incurable. As there is treatment available for Heptitis B. Even if you argue that concieving is a problem, there are other methods available to ensure it. Even if you argue it will spread to child, there are adequate measures which can be taken to prevent it. If s*xual gratification is the only reason to seek divorce, you can as well use contraceptive methods. Moreover hepatitis B does not fall under venereal disease as per amendment of HMA 1976.
However if some advocate proimses to get you divorce on above grounds, he is lying, by end of it , petition will be dismissed.
I advise you to stay put and dont interfere in the couple's life.
One other option is take mutual divorce by paying hefty one time alimony to wife and co. She go her way your brother go his way.
Your brother might have had s*x atleast once with her (probability of not using contraception are high on first night). So better get checked regarding your brothers health.
But just also think from girls and her parents standpoint. Now that you guys came to know that she is Hepatitis B positive, you will leave her? What if same had happened to your brother?