Am I being overprotective ?

My 4 year old niece is living with my grandma in a different state. My mom has a female friend that really likes my niece. She’ll often times hangout with her one on one without my mom there. I don’t know why but this makes me feel uncomfortable, and I plan on telling my mom that I don’t want her to be alone without her there. I know I’m not my niece’s mom but something feels off.