Yet sometimes we also need to make those mistakes to learn that they are something we don't want to repeat. I wouldn't say we shouldn't regret, but I do agree that there is something we can learn and develop from the things we regret doing, so we shouldn't dwell on them to our own detriment, only learn from them. What's done cannot be undone and I'm sure your sister only brought it up out of concern for your son. From your son's perspective hopefully he can learn that no harm was meant by it, only concern from both you and your sister.