Since your mother in law smokes three packs a day and you spend almost the entire day with her, you are being exposed to a high amount of second hand smoke. Passive smoking
is harmful for your body and even increases one's risk of developing lung cancer
even if you are a non-smoker. So it is quite possible for you to feel sick. Spraying air fresheners won't help much. Try opening the windows and if that is not possible, try to avoid being in the room where she smokes.