If you do buy the Paranormal Stuff Pack for Sims 4, you can have a romance with Bonehilda. Summon her and build up a relationship, just as you would with another sim. Bonehilda is woohooed with, but, without cheats, you can't marry her or try for a baby. To do so you'd have to cheat to add her to the household. If you do so, you'll discover that Bonehilda is really the illusion of a skeleton-- when she bathes, she's a regular sim.