$100 present? Can I be your sister in law??? J/K. A gift certificate sounds like a nice idea for someone you don't know what to get for. Maybe to a mall so they can have a wide selection to choose from?
Does she like bath and body products? Bath & Body Works is having an amazing sale right now. You could do a REALLY nice tote filled with products for what you want to spend. That's just an idea if you want to actually give her a present.