No you cannot catch it from her. I got it in highschool and my sisters refused to even use the same bathroom as me out of ignorance. However it cannot be contracted through a toilet seat (considering that you are talking about genital herpes) or using the same shower. The only way it can be contracted is through contact of an affected area to an unaffected area. Make sure your niece knows that she has to be very sure to wash her hands after using the bathroom during an outbreak because she could spread it to her eyes or to areas of her face if she has gotten any fluids on her hands. My doctors made sure to be very clear with me on that point. Just focus on getting the whole family educated about it because not knowing and assuming that she is "dangerous" or "dirty" in anyway because with her unfortunate situation can cause severe emotional damage to her. I know i was deeply affected whem my sisters called me things like a whore and slut, because i contracted herpes from the first person i had sex with.  Just make sure she is loved and cared for by everyone. Also keep everyone conscious that it may not have been her fault and that no one deserves to have an STD.