I don't see this information in your post:
How much of an inconvenience to your SIL is this? Is your home convenient to her work and school? Will she have to lug a lot of work/school materials to your home to do what she needs to do? Is there adequate work space for any homework she may have?
I would take any inconvenience (or niceties) into consideration as well. When we have someone stay at our house for something like this, we make it like a spa week for them basically. Their own quarters (guest room and bath), fresh linens, spotless accommodations, the fridge stocked with some of their favorite yogurts/frozen menu items, the pantry with snacks, and a fresh bathed dog.
We leave them all the internet access information so they can go online from their laptops, watch movies on instant netflix, etc.
So, even if you aren't providing "food" for her, I think you should provide some "goodies" for her. She is, in fact, doing you a favor, even if you are paying her for it. Not having to deal with loading up the dog and delivering them to an off-site kennel and picking them up on your return is a convenience to you. And probably a stress relief on your dog to be able to stay in its own familiar environment the entire time.