Is a wife considered forbidden to her husband if he said to her in a message: "You are forbidden to me just as my sister became forbidden"?

The more likely view is that the matter depends on the person's intention, and whether by the utterance he intended ẓihār, or ṭalāq, or an oath. An angry person is accountable as long as he comprehends what he is saying. Writing does not entail its content, whether ṭalāq, ẓihār, or an oath, except with intention. One must be wary of anger, acting upon the Prophet's (peace be upon him) وصية (counsel), as it is from Satan and leads to many corruptions and evils.