Wow… that sounds like an intense and emotional moment. The way you phrased it makes me think this envelope could carry something significant — maybe a letter, a confession, or something related to their future.
Given that he specifically told you not to open it in front of the girls, it seems he wanted to control when and how they learn whatever’s inside, probably to protect them or you from an immediate emotional reaction.
You’ve already done so much by raising them for 15 years — this might be about acknowledging your sacrifices, passing on responsibility, or revealing something personal to them.
If you want, I can help you think through the best way to approach opening it and handling whatever’s inside, so it’s as gentle as possible for you and the girls. Do you want me to do that?