sister

If you were adopted and you have a little sister, is it still incest to fuck her?

Not related in the least, just adopted.

No risk of her giving birth to gargoyles

Etc.

>>1
Legally?  No.  As long as you are not blood related, there is no law prohibiting sexual intercourse with an adopted relative.  Incest laws apply to those not legally allowed to marry due to blood relationships.

Socially, however, you'll be labled a freak.  If anyone finds out that is ;)