Female sim Piper is stalking my male sim

I think what I would do would connect the two as brother and sister. The back story would be that she is his long lost sister. For story purposes, it would answer why they are friends, but never romantically involved, and why she is following him around. She hadn't quite got the nerve up to tell him this information.

This can be done by temporarily moving her in with his household and then editing the family. Just make sure you have the cas.fulleditmode cheat turned on.