Technically the female in this case is not biological and thus no real difference than a "random" female. The pure fact of her being raised and brough up with you and treated as a "sister/family member" does not negate the fact that she is biologically differenet than you than Pam Anderson.This is classic nuture vs. nature but this should not let feelings and overall created human thoughts to involve the option to have intercourse with a woman; "related or unrelated." You should be able to have sex with you foster sister, regardless of morals.
I feel that sex with a sister, biological, foster, or step, destroys familial relationships and can cause severe problems among members of the family. Most relationships that involve sex between younger people will generally break apart. This is acceptable in most cases as the relationships can simply be broken off and the couple can avoid each other. Avoiding a sister is another story. Not only can this lead to tense relationships within the household, it can cause marriage problems to arise with parents who will most probably feel differently. I feel that the disadvantages caused by inter familial sex outweigh any possible benefits due to marriage trouble and domestic tension.