I'll be interested what others say about your second date letter. Can you just arrange for your son to go to your sisters and not even bring up sleeping together? Would it be normal for him to want to see his aunt and cousins? Has he stayed over there before? Saying you have no intention of sleeping together would make me feel like you really did. Telling her she is attractive and you value her feelings is important, but I am just not sure you should do that in writing for a second date request.
What about a simple clever coupon "good for pizza and dancing"? Redeemable such and such a night.
