How do you answer?

I don't think there's anything wrong with saying, " I haven't been in touch with SIL in a while." It's a simple and smart response. It's reasonable and there's nothing hard to believe about it. If you tell someone that you haven't spoken to her lately, the person can't hold you accountable for not being "up to date" about the situation. Chameli, trust me.........I EVEN KNOW about sisters-in-law who only LIVE within minutes from each other and they STILL don't know what's going in each others lives because they're so busy with their own chores. Your response was fine. And it's no business of a non-related person to interfere in your SIL's private life anyway. If the person persists in being nosy........shame on them.

Other responses you can give are:

"It's her own personal matter, I haven't been told anything about it."

OR

"My family considers this a confidential matter. However, I appreciate your concern in asking, but Alhumdolillah everything is fine." (This way the person understands that the matter is private and backs off).

OR

Smile at the aunti and give her a vague response. "My SIL's doing well, Alhumdolillah. Everything is fine. She's a smart and independent girl and she's handling life to the best of her ability like the rest of us."