good points Lynne. I have also heard of and would suggest that you take some of the crib sheets/blankets that she has slept on for a while so the smell is familiar to her.
My DS was very little when we did the pack n play so it didn't phase him very much. You don't have to follow this but this is what I would do......
**First, unpack your 'pack n play' to air it out, then when it doesn't smell as much take the bottom/flat section and try to somehow fit it in the crib underneath her bedding (to get used to the feel of it).
**If or after she is comfortable with that, take the piece out and the bedding and let her sleep in the pack n play in her room for that nap time that she will be at your MIL's place.
**After she is comfortable with that then take her to another room in the house and let her sleep there.
**Again, when she is used to it, see if your MIL will let you come over for a day or 2 during her nap time and let her sleep there. If she sleeps well then you should have no worries. A bonus would be if you like your MIL, you can sit and have a visit with her.
This would be the transitional way to do it, but if you don't have time for that you could just jump straight in to it and take her over there and do PU/PD until she naps well.
Hope that was of some help.
Good luck,
Carrie