Just let them all know how crappy you are feeling. I would think that it would be okay to have her let herself in, she will be tired from the trip too, and may welcome not having to stay up and visit. I would have everything ready, and leave a note, you might also leave her a bottle of water and a snack on the counter or in the fridge and tell her about it in the note. She might not be comfortable on the first night, making noise russling around the kitchen, so having something ready would be really nice (just cheese and crackers would be great I would think, and not a lot of work, or even jsut make sure the fruit bowl is full and stick a chocolate bar or some treat with it).
I would also be honest and ask her about what time she might arive, late late to her might be 10pm or 2am. Then go from their about what your plans are. Have a fun weekend, it will all be good, don't stress too much, they are here to see you and not your house. It will be a mess after the first 5 hours anyway.