First of all, get a large red marker and circle the day this happened on your calendar. After all the work, tears, guidance, threats, and bribes, your girls have finally started to act like the best part of sisters. I would do...absolutely nothing about this. It's not like your older daughter lied about what she wanted the money for--you didn't ask, she didn't tell (maybe a future military career, ha ha) Also, although you did tell your younger daughter that money was for her lunch account, you didn't tell her NOT to loan it to her sister! (I'm sure you realize the importance of these distinctions at their ages!)
On the other hand, I would not give your younger daughter the money to replace what she loaned her sister. Until your older daughter pays her back, it won't hurt to bring a brown bag lunch to school. This reinforces the lessons of lending and borrowing, and builds the girls confidence and trust in each other.
As a single mom, I know how hard it is to wonder and worry about whether or not you're doing as good a job as you could or should. Relax. Whatever you're doing is working. As a former member of the sisterhood, I'm officially proud of you!
D2