Is it normal that when i don't know someone i think they are fake/mean?

Whenever I see someone that I have never talked to before my brain automatically processes them as rude, mean, and faking nice personalities. However, normally they are not. Whenever I try to express (in private) that I feel someone is being fake, everyone always tells me not to be so judgemental and rude. It's almost like no on else see's what I see. For example, I am going on a mission trip to Haiti. My mother is a teacher and told on of her teacher friends who is a strong mean personality that she should come. The lady is bringing her two daughters the same age as me who are in fashionable clothes and who are pretty with her. When we were in a meeting for the trip, my Mom offered to give their family a ride (which is 4 hours, plus an 5 hour plane ride) with us. These twin girls are always talking and snickering to each other well looking at me. My Mom just goes on and on about how I should be a nicer person and that they are one of the two sweetest people in the world. They keep glaring at me and whispering. Me and my Mom and Dad were in the car and I said "Those girls don't seem to like me" and they both say that I need to be nice and that I'm probably the one bullying them and that I should think of others.