I am not sure if you are asking if you should tell her you are TTC or if you are asking if you get a BFP first if you should tell her before announcing to the rest of the family? I would not tell her you are TTC unless it is something you would want her to know anyway. If you do get a BFP first, I do think it would be most kind to tell her before she can hear it from someone else.
We're thinking of announcing to her before anyone else. No one knows we're trying and I'd rather no one knew. If she gets a BFP first we'll probably just hold off telling. We're *hoping* to make it to our mid-pregnancy ultrasound before anyone finds out. I'm going to have to invest in hooded sweatshirts and a billion excuses to avoid extended visits with family to pull it off though!