Problem with photographing family because her mother also has brown teeth from betel?

Bigger problem. If your mother in law chews betel she as bigger problems then just stained teeth. People who chew betel have a much greater chance of developing oral cancer then than those who don't chew. I would urge you to ave her evaluated by a dentist. In the mean time you can use photoshop to whiten her teeth in family photos.
Ortho phosphoric aci. Superphosphozyme by biotics will whiten teeth. Use a qtip with few drops use on teeth and rinse off using water with little baking soda to remove any acid (or just rinse thoroughly).