Oh it's hard isn't it? DD is just getting past a few very tough months of this and it just broke my heart / drove me insane (in equal measures!!). I understand it can come up again at about 18 months so I'm sure we may see it again.

I found there was not a whole lot I could do about it, but then my little one was too young to understand a lot of the language I was using that may have helped. Being older, your DD understands a lot more so here's some things I can think of that MAY help...
- You should say goodbye. It might make her cry at the time, but it's very important that she understands that you're going, and that when you go, she gets warning. If you just disappear, this would be very frightening for her and possibly make her fearful in the future that Mummy could just suddenly disappear for 2 weeks.
- Start preparing her for it now. Make a story about what's going to happen. Take photos of you and DH, a plane (or however you're travelling), a hotel, your ILs, fun suff at your ILs house, where she will sleep there (if you can) and a picture of you being 'reunited'. You can tell her the story (just as an example) "Soon Mummy and Daddy are going on a holiday. We will go on a plane and stay at a hotel. You are going to have a special holiday too with Nanny and Poppy. You will get to play with... You will sleep... After 14 sleeps at Nanny and Poppy's Mum and Dad will come and get you" (You get the picture I hope?!)
- Make a chart of how many days you will be away. This could be a card with 14 pics of beds stuck on (velcro or blu-tak), and the last picture of you. Every time she goes to bed she can post one picture in a box or an envelope and see how many more 'sleeps' 'till Mummy and Daddy come home.
- I think it'd be nice if you call her, but it's hard to predict if this might make her more upset. I guess you could make it more predictable and part of her routine at the IL's if it were to happen at the same time every night (say after dinner), so that it wasn't too much of an emotional shock to hear from you out of the blue and then not know if/when it's going to happen again. If it's going to happen, you could put it in the story - "Mummy and Daddy will call to say 'I love you' after dinner"...
- Has she got a favourite toy? You could give it lots of cuddles together and maybe put in the book - "when you feel sad and miss Mummy, you can give bear a big hug because he's got Mummy's cuddles inside. You can give Nanny a big cuddle too." You could even sleep with this toy for a few nights so it smells like you.

That's all I can think of for now but if I come up with anything else, I'll come back.

FWIW, I suspect while she may get upset, she will probably adjust pretty quickly. My DD was absolutely inconsolable when I first left her at childcare - it was AWFUL. But, after a couple of goes, she'd have a little cry for a few mins after I left, and then settle down and get on with things. She'd cry again when I came back as if to say 'oh Mummy, it's been SO hard!!', but from the type of crying, I could tell it was more for show than real distress IYKWIM. Your DD may find the first few hours /day pretty tough, but she will find her rhythm with the ILs. I think it's great that MIL is willing to take on what you do with her, as this will help keep things as familiar and 'normal' for her as possible.

Good luck and have a WONDERFUL and relaxing honeymoon. xoxo