It sounds like all of your children have done well with their education. For that, you and your spouse are to be commended. As for the leftover funds in DD2's account, why not have an honest, adult conversation with her? Ask her what she might want to do with the funds. You could mention that the money could be used to help her siblings pay down their education debt. But, as she is an adult, the final choice should be hers. If she doesn't choose to help out her siblings, they should never be told about the conversation. It could result in bad feelings.
Be upfront and honest with DD2. Lay out her options and let her choose. Whatever she chooses, let that be the end of it. Once the choice is made, move on.