U have to check whether there's displacement on both sides of the crack. If there's displacement even for 1mm, the stairs is not stable and will collapse! if not, treat it like hairline crack. Since each time u walk on the stairs, there's going to be vibration, solid filler may not work for your case, as the line will reappear. U can try mixing silicone base filler with cement (or some flexible filler), and fill the gap accordingly, it will work.