A.R.
How her grandparents parent shouldn't interfere with how you parent at least when grandparents aren't around. I have found that if I sit down with my child and give a hug and explain why her behavior is inappropriate and then put her in the corner she is a bit calmer. Initially she continued to scream, yell, kick and throw things but it does get better with time if you're consistent. This morning she just sat there and waited for the timer to go off then went to her "brother" and hugged him and told him she was sorry for hitting him. I think it helps if she sees other children disciplined in the same manner so if you have friends with children the same age...
The sleeping will be much more difficult for you and grandparents. Grandparents are going to have a difficult time to change the pattern now - after all, what child would not prefer to sleep with someone. BUT they can change the habit. What I did was put a chair in the room and read a book while the children were in bed (I let them have a book or a toy and told them they had to lay down with the book or toy) and I stayed until they fell asleep. (The 3 yr old boy spends weekends with dad who likes to sleep with him and has different ways of handling discipline) (I found with the discipline I have problems when dad comes to my house - then tantrums... so I have to be firm with boy and dad -- my house my way.)
I have a 29 yr old son, 4 stepchildren, 9 step grandchildren and now I am raising 3 foster children ages 3, 3, and 8. (the 8 year old has downs)
I hope this helps. Don't worry it gets better as they grow!