The scenario you describe in your request is very sad and serious. For a child that young to be not just disrespectful but that abusive is shocking. I assume her mother is also deceased and that's why you have been raising her.
You said "her behavior is getting worse", and it's been 1.5 years already, what is concerning, considering it is an old pattern which is just escalating more and more. If there is no abuse nor neglect at home or at school -where she is doing "good", then what could be fueling this serious and chronic problem?
Have you spoiled this child at all? The measures you have taken seem all good but ineffective. You mentioned this child has a laptop, cellphone and other things that could be ok but negative if a child keeps such abusive pattern most of the time.
If discipline works fine for a little while and a child gets back into the very same behavior, then the boundaries must get clearer, consequences reinforced, increased and implemented with full consistency. When care takers do not do things this way they end enabling the destructive behaviors and manipulative nature in the child. If spoiling has been there as a way to compensate the lack of parents, things get much more serious, since it is from there that any distortion could develop in a child's mind, heart and personality.
Here we are not talking about a child with depression, anxiety or other mental health problems as the core concerns but, being very disrespectful and abusive exclusively with grandparents and siblings while being totally well at school , with friends and when visiting family. This seems to be the case and I urge you top get professional counseling in order to improve parenting and work on making adequate changes at home and in your relationship for this serious distortion to end and for this child to have a healthy development and a fulfilling adulthood. Individual counseling for her plus collateral-family sessions to work on it would be necessary too.
Hope it helps.