This is one of those great "what if" kind of questions that really depend so much on our own reading of the situation and of the characters involved. What is clear, however, if we look at the end of Chapter Seven, is that Victor does not feel able to tell the truth of his story, which would automatically clear Justine, because he thinks that Justine will not be convicted of this crime and also he considers his story too horrendous to share. Note what he says:
My tale was not one to announce publicly; its astounding horror would be looked upon as madness by the vulgar. Did any one indeed exist, except I, the creator, who would beleive, unless his senses convinced him, in the existence of the living monument of presumption and rash ignorance which I had let loose upon the world?
Although I personally believe we cannot excuse Victor from some part in the responsibility for Justine's death, as his story could have cleared her, at the same time, let us think for one moment what would have happened if he had shared this story. It is clear that it would have been difficult for Victor to convince his family of the truth of his words. The story would have appeared so strange and fantastical, and given the extent to which Victor had studied and worked so hard, they would no doubt have concluded that Victor had lost his mind from overstudy, and sought medical help. Therefore I think their reaction would have been one of disbelief and incredulity.