Jes,
I took care of my MIL for only one month when she was ill and that was so hard! People think babies are hard, let me tell you, I've done both and babies are easy compared to having an elderly person in your house that is ill.
I admire you for taking care of others and you should NOT feel the least bit selfish. A person can only handle so much of taking care of sick and elderly people in their home 24/7. I hope your step MIL gets well and hope you find lots of support with helping her.
Lisa