How to get a ghost back to her beloved family?

With Seasons installed try moving the urn into Becca's inventory and see if you can gift it to Lien's family.

Otherwise move in a townie with Becca, put the urn into the townie's inventory, move the townie in with Lien's family and put the urn on the lot. Then you can move the townie back to their original household. For this approach use a townie from a household with several sims, so that no money gets added to Becca accidently.