I know the feeling - its terrifying! I remember after I dropped DD#1 off at my sisters for my first night away from her - I cried for almost an hour! not good when youre trying to get ready for a really important work dinner... anyway, she was 20 mths then - and she survived. She was happy as larry when we went to pick her up in the morning - and it was soooooooo nice to have a nice meal and a good sleep (well, we didn't really sleep that well )

Have a lovely time! Can you ask SIL to text you at certain times so you aren't anxious? theres nothing worse than sitting at dinner with your partner and constantly checking your phone for messages!