ok heres what i got, i have a two sims that are brothers one died of hunger in the young adult stage. i left his grave at the residence and waited for a long time. now my other sim (the one thats alive) is now a adult (was a teen when his brother died). now every once a while, the sim will come out of his grave but he is not controllable.
theres my case scenario i have going now
btw my alive sim works at the grave yard in sunset valley as a eptiah writer.
Ok, first thing you do is get your friendships up with eachother, the ghost should show up in the other Sim's friend list. Then use the cheat: testingcheatsenabled true, go to the ghost in the friends listing and raise the value up so they become BF's.
If that doesn't work and you still don't have the Science quest, try working on your logic skill and then you should get the quest.
When you get the quest, pick up the gravestone and place it in the Sim's(who got the quest) backpack, then go to the Science Building, after spending about an hour or 2 (sim time) in the building your ghost will come out before the other sim will, and you will be able controll them once they exit the building.
I hope this works for you!