I am an experienced nanny of over 10 years for multiple families. I would say it doesnt hurt to ask your sister in law first. As far as what you should offer her I think it depends what hours she'll be at your house for, you said you work nights is it all night or like 6pm-12pm? You could pay her hourly or a flat fee per night. Also should take into consideration what you can afford to pay and what going rate is for babysitters/nannies in your area. If she says she's not interested I would suggest you check out Care.com to help you find an overnight nanny possibly.
Is it possible for one of you to change the hours that you work?
So child cares are overnight
Most child care is during the day. But I've seem familys look for overnight nanny's on care.com before. I myself work during the day but maybe a couple times a year the family I work for will ask me to do an overnight.