The drinking could be doing it. It could be too that they are having a higher carb item without even relising it. Alot of hidden carbs and so on. Encourage your mother in law to start keeping a daily food journal. She can do it on paper if no access to the computer. Maybe then at the end of the week you can sit down and go over it with her looking for hidden carbs in things like the drinks, coffee, cream, eggs and so on.

As for your mother, all you can do is keep trying to be a good example. When you go to her house bring a dish along to share with the group that is LC. Hide an Atkins bar in your purse to have later while everyone else is haivng dessert. Adults her age are really hard to change habit wise, and I think for many people kids are the last people they will take advise from. The more you push her, the more she may try to resist. You can also invit eher to dinner a few nights and serve an LC feast so she can see how well you really do eat and that you aren't completely depriving yourself.