This 8 Year Old Girl Constantly Fights With Her Younger

This 8 year old girl constantly fights with her younger sister, she has tantrums, she gets very irritated when she hears someone scratching nails ( like when you peel nail polish off your nails), she takes a long time to eat (even with a timer) then she worries that she wont get more. Her mom or dad have to tell her when either one is leaving or she cries hysterically. She has 2 friends. she has motion sickness when in the car. therefore she doesn't want to go on vacations, she likes things in a certain way but yet her room is a mess. Consequences don't seem to matter. The school wants to put in group therapy but didn't really say why. Then the therapist went on maternity leave, Could this stem from physical disorder?, mental disorder,? an imbalance, ? Behavior disorder? not really sure what direction to suggest to the parents. Can you please share what you think? thank you