Torn... To tell or not to tell?

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My stepchildren's mom recently was arrested for a bouncing check case. She is now out on bail. She has confided in me and asked me to promise not to tell the kids. Naturally, the kids wouldn't know about it as long as no one tells them since my husband and I have full custody of them and she doesn't really visit like on a regular basis. I did tell my husband about it though and he also felt that we wouldn't let the kids know. I understand the fact that letting the children know might cloud their judgment of their mom further, worst it might affect their self-confidence and self-respect (the boys are 18 and 17, and the youngest is 11, a stage when kids start going through difficult phases). However, based on experience, no secret will remain a secret forever. What if the kids eventually find out and learn that I know about it too, but didn't tell them? The kids trust me and I wouldn't want to do anything that would make them hate or distrust me. So, should I tell or shouldn't I? Do you think they have the right to know?