Sorry, can't really help you with this one, as your sister's privacy is too high to allow me to see the Edit tab. But since your parents' names are in red in the biography, that means maybe you made a typo in their Wiki-IDs, so the system thinks "no such person" and won't link. This may be the reason you can't add your sister to an incorrect Wiki-ID, as well. But this is just a guess, because her privacy is too high for me.
