I made a small cemetary for my Sims in Windenberg under the high wall and one of my Sims took her mum Hazel's urn there along with the family so that they could mourn her and engrave an epitaph on her headstone. I have since found it useful for another family whose dad's urn I couldn't trace. They went there and mourned Hazel! It seemed to satisfy them. (I loved the original example on the headstone - "I told you I was dead").
I made the plot Generic and given it a Haunted, dog friendly and Gnomes traits to keep it tidy, plus a public toilet and hope the ghosts can haunt there. I'm not sure if they can haunt outside their family home.
A strange thing-- On going back to another family who've been dropping like flies (another eight). There are now three named headstones in their memorial plot in their large garden and I cannot remember putting more than two in! Who did that? :o