Im Scottish and married to an Indian, is it right to keep his sisters secret ?
To be honest this is a difficult situation for you. I have some understanding of the Indian ways of life and find that the family as a whole seem to be quite modern in their thinking, although the girl is going to marry this boy when he finished college etc, it seems to me that the girl wants to follow tradition as much as she possibly can, the fact that she has consulted you, her Mum & her sister shows some of her westernised side, but as far as I can see this she wants to do the traditional side of her culture as regards her Father & Brother, please allow her to do this the way she wants. Support her as much as you can, but don’t lay yourself & your marriage out on the line to danger.
Your sister-in-law did NOT have the right to endanger your marriage by asking you to keep a secret from your husband. It is YOUR marriage, and it comes FIRST. Do what is right in your marriage. If that turns out to be bad for your sister-in-law and her boyfriend, it will maybe the fault of your husband, and very probably the fault of your sister-in-law, but CERTAINLY NOT your fault. Trying to work out what is right and wrong by the consequences of what you decide is a useless guessing game, and too easy to guess wrong. You don’t need to start. Your husband comes before your sister-in-law, and she should have known that before she let you into this.