I helped my brother in law financially for his Ph.d and in dissertation doc, he simply mentioned me as his brothers wife. (not sister in law). is that normal to not call me sister in law after 6+ years of marriage to his brother? I dont see why it would be weird to call me sister-in-law
As far as I know, there is no particular etiquette or customs regarding this choice of words in thesis acknowledgments. So the answer to this question does not depend on any peculiarity of academia.
In view of this, I suggest you to ask your question on [interpersonal.se]; they may have more relevant advice.