alright my quick n' dirty three-generation household with a playable ghost (ghost + human married, each of them has a child and a grandchild each) reveals this. The definitive step-siblings guide, lol.
1. Adult step-siblings can't be romantic with each other while still step-siblings. No real way around that unless their parents break up by death or divorce.
2. The children of different step-siblings can be romantic with each other just fine. They can even go steady though I noticed that the family tree won't show their portrait, probably for visual appeal instead of anything serious.
3. The step-siblings themselves don't see each other's children as related either. If you really wanna go that route.
4. Now here's the important part for anyone playing human x ghost couples and interested in step-child relationships (ok): when a ghost fades into the netherworld, their marriage remains intact. You probably could have the living spouse pursue another steady relationship instead and break the tie. Or try using the Clean Slate reward. (Woops didn't see your post Chubling...confirmed by another party then?)
