How to untangle my sisters hair?

Okay so my mom was really stupid and didnt brush my 3 year old sisters hair in a long time. My sisters hair is really curly so thats what makes it harder to brush. When my mom tries brushing it, my sister cries ALOT. She has tangles starting from her cranium down to the end of her hair. When we wash her hair, my sister cries to much when we try to untangle, so can someone please tell us how to untangle her hair with no crying; or atleast a bit crying ?