Interpersonal relations during school age are taking turns and fresh entrants in family take long time to get accepted and assimilated. You will have to have infinite patience with your kid who had solo reign for quite some time !
There is no need to rush to any therapist just from this tiff. When there is danger to someone physically (does not appear from description) then only psychiatric help is to be sought.
Please continue to have dialog with her one to one. Also try to make them play as a group, under elders' supervision and encouragement.