Is the husband's statement to his wife after a quarrel, "You are like sisters," considered Dhihar, even if he did not intend Dhihar?

If the situation is as you described, then no ẓihār has occurred from you, and nothing is binding upon you, because you did not intend ẓihār. As for the fatwa you referred to, stating that kināyah (ambiguous divorce pronouncement) takes the ruling of ṣarīḥ (explicit divorce pronouncement) with the presumption of dispute, its place is when intention is absent, or when ruling judicially, not religiously. So, if he intended something other than divorce by the kināyah, it would not take effect, and if he claims that, he is to be believed.