The LiveStrong organization has a great guide and planner. http://www.livestrong.org/we-can-help/guidebook/
As a caretaker be sure you both have "Me" If you don't take care of your self and your interests you will burn out and both of you will want to kill each other.

All great answers! I found the chef part to be the most difficult, my sister would crave some food, then 5 minutes later decide she doesn't want it. Small portion sizes, homemade broth, boost drinks.
A crossword puzzle or sudoku book to distract her from the chemo. Just being there and letting her vent. Lots of hugs. Keep the doom and gloom relatives and friends away from her.

Lots and lots of great info here. Much of what you do will depend on what your sister-in-law needs and wants. Be flexible and cheerful but if she wants to be serious, be prepared for that as well.

As LilyMadeline said, please schedule some time for yourself too. Caregiver duties can be very taxing and you will need to recharge your batteries.

Finally, if your sis-in-law is likely to lose her hair, you may want to request a free scarf here: http://www.goodwishesscarves.org/donate.html. They're beautiful and come with the story of a survivor. I found it very uplifting.

Thank you for your generous and willing spirit. Hugs!