Continuing My Nature/Nurture Thoughts

Our opinion of nature vs. nurture depends on how we examine specific examples.  If we see only the practical applications we can formulate different theories than if we trace concepts to their origin.  Take, for example, Pinker's explanation of identical twins.  He states that different personality traits are determined by prenatal factors such as blood flow or toxicity.  This had validity in my personal opinions, however I want to cite an example to offer where an argument could be made.  Knowing that often times twins (identical in this case) are born with different weights which can impact their development and needs- does this make all nurturing instances moot?  If twin A is born 7lbs and twin B is born 5 lbs, is it safe to say there is an increased likelihood twin B will require more extensive nurturing and regardless of success with that nurturing problems will still exist due to natural origins? This is a very difficult debate to argue, as we have concrete evidence that natural instances determine characteristics unique to each individual but we also have become accustomed to being told that parenting and nurturing are very important in human development. While I don't like to classify myself exclusively ascribing to nature, nurture, or holistic interactionism- I will say that facets of each exist in science, culture, and family.