Honestly...you should be worried. Your MIL you should be ashamed of herself!!! You can get herpes simplex (cold sores) whether or not there are any signs of a cold sore. It is a virus that you can treat/manage, but NEVER get rid of.
The virus will always be present in the body, even if a person does not have reoccurring outbreaks. It is false to say that because a cold sore is not present that a person is unable to pass this virus along to others, but the virus is more likely to be spread when sores are present.
Once you have them you will always be at the mercy or certain triggers: stress, lack of sleep, sun, nuts, certain fruits, etc. Honestly...your MIL needs to be a bit more educated because they are a horrible thing to pass along to someone you love!
Even though many people have the virus that causes cold sores, there are ways to minimize catching the virus or spreading it to others. Avoid kissing while there is an active sore. Don't share objects, such as lip balm/drinks, that touch your lips. Wash your hands often, and wear sunblock, since sun can be a trigger for an outbreak.
Her best bet is to stick to hugs. I think you should tell her so...it isn't worth being wrong about. I have had the stupid things since a boyfriend gave them to me when I was 19. They are MISERABLE, and every time I stress about work or down a bag of almond M&Ms they are there to remind me.