Extremely knotted hair? How can we fix it !?!?

Okay so my sister has extremely knotted hair she brushes it every day but doesn't brush it to the point where all the knots are out so today i seen huge knots in her hair and i told her to brush it out so then she tried and it was extremely hard! So then she asked my dad.. then he has been trying to brush it out for over an hour and we even used like half a bottle of knot spray then we dunked her head in water and tried brushing it out again... she took a shower before that... we need to find a way to untangle her hair without having to cut it really short! Please Help!!! ( no rude comments)