Hello and welcome to HCM,
I can understand your concern regarding the suspicion of TB in anyone else in the family.
You can wait for the report of the lymph node biopsy
and if that confirms without a doubt that it is TB then any person who was in close contact with your wife's sister in law should be tested for TB.
The test can be a simple blood test
or even X ray
but it is better to be safe than worry later on.
Also even if she was diagnosed as TB, she may not be the primary source of infection and it can very well be anyone with whom she was in contact.
Having said all these, let me assure you that TB is treatable and if you stick to the medication it can be very well cured.
take care and get back to me if you have any other doubts
Dr Sriram Nathan