^C’mon Masti, that was uncalled for. I know some people who feel so uncomfortable about even telling the smallest lie. That’s just their nature. And they would rather word something in a way that would not be considered lying but at the same maintains their privacy. Everyone’s different. However, I understand your concern about keeping marital affairs private. Because I remember you had long ago created a thread about keeping marital issues private, so I know it’s an issue that’s very important for you 
Chameli…you don’t have to completely fabricate or make up a story about your SIL. Just give a response that indicates the matter is personal. Examples:
“I don’t feel like talking about it.”
“I don’t feel comfortable discussing my SIL’s affairs. You can talk to her about it.”
“This is a confidential matter regarding our family. We prefer not to discuss it.”
Or give a vague response and divert the asker. **“My SIL is fine. How’s everything going with YOU/YOUR JOB/YOUR KIDS?” **The asker will get the hint that you don’t wan to talk about it.
And NJMasti is right in being stern about this issue. Marital relationships are delicate. So, you should be practical and sensible because the smallest mistake on your part can ruin the relationships between YOU and YOUR HUSBAND and YOUR SIL.
You’re not obligated to tell an outsider from the family (no matter how close they are) about your SIL’s problems. Nor should you be feeling guilty for not telling this friend about your SIL’s personal life. First of all, it’s your SIL’s personal matter. If anyone has the right to talk about it to others…it’s your SIL…not you.
And Chameli, you have to decide what is more important to you at the end of the day: Your marriage OR disappointing some female friend? If you value your marriage, then respect the privacy of your husband and his family.
From now onwards, don’t tell ANYBODY outside of your in-laws (including your own parents/siblings) about your in-laws personal matters. You never know who might start spreading the word around. And your husband will end up getting into trouble for sharing his sister’s secret with you.