It could be worse. What if your husband/in-laws had bought you the bangles…and questioned where they were? Then what? (Don’t tell your husband about it…it’s a matter that’s between you and your immediate family).
Your mother is more worried for YOU than the gold that her younger daughter may or may not have taken. So, your mom has your priorities right. I understand that gold isn’t cheap…but you may receive it as a gift again in the near future. Chock it up to a learning experience…if you know what your sister is like, then don’t store your jewelry in a place that is accessible to her.