I have nothing new to add, just questioning the dates in Laura's response. Also, I believe Sally Warwick was a close friend of Agnes Lee, not Mildred Lee. If you read the book The Lee Girls (1987), the timing seems to suggest an engagement in the summer of 1863, but as the date of the wedding was postponed again and again, Robert E. Lee wrote to his wife in early August 1863 that maybe they would not have a "new" daughter after all. Then in Novemeber 1865, Agnes traveled to Richmond to attend the prenuptial and wedding festivities of Sally Warwick.
May I ask why you are seeking this info? Because I too am curious about this episode and have not been able to find out much about it.