Shoes inside the house..

We don't allow shoes inside our house. I've got really young kids and having nasty shoes in the house is disgusting with them picking stuff up etc

My MIL came over and she was completely barefoot. She said she left her shoes in the car. I was just surprised and confused as she just came right in and I said ooookayyyyy. She clearly doesn't understands the point of no shoes in the house!

She always appears at the porch with her shoes already off. She usually takes them off and stands on the ground and waits for the door to be opened. My point is I don't like bare feet on the ground and then entering my house. It's like her shoes were on the whole time. I want her to remove her shoes and step into my house so her bare feet never make contact with the ground. She had even walked all around my back porch barefoot and came back in the house once.

How do I talk to her about this without any awkwardness?? Please help me with the wording...... I don't want to hear if you agree with me or not about having shoes in the house. Just need help with how to tell her.

(Please, no rude comments about me being OCD about dirt and germs in my own house or something...)