1) That should be fine - if memory serves, my vampire heir had a half sister.
2) If you can do it without the heir and best friend ever achieving best friend status, you should be fine. However, the chances of that happening are pretty slim - and best friends must remain unique the entire time. Personally, I wouldn't chance it.
Edit: Yup, Lilith had a half sister named Jess (Jess was also a vampire, as was their father, Shea; Jess's mother was a witch).