Plus ones at intimate wedding?

My fiancé and I are having an intimate wedding of just close family (17 guests total) but my fiancé’s mother wants to bring her friend with her. I’m not sure how to feel about it because this person would be a complete stranger to me, I’ve never even seen them before. I’m not inviting a couple close friends of mine because we agreed to just do family. But at the same time I am allowing my siblings to bring their significant others (my fiancé has met them multiple times At get togethers). With such a small wedding I think it will feel weird having a complete stranger there but I can understand my future MIL wanting to bring a friend because out of our 17 guests she only knows 4 people. What do you guys think?