@zaneeta as a soon to be parent, it would really bother me if one of my family members decided that they could handle the topic of sex with my kids better than me, so please don't do that.
However, I am all for having a great relationship with your niece, encouraging her to do well in school and in relationships, making sure she always feels comfortable talking to you about anything, and knowing that you always have her back.
But I would let her be the one to bring up any sex topics, and I would probably say, is this something you want to talk about with your parents? If she feels uncomfortable, by all means, answer her questions. But don't leave her parents in the dark, or just assume you can do it better.
I have a similar situation with my nephew. I have such a huge urge to keep him for a weekend and just tell him everything I don't think he is hearing. But he is not my kid, and therefore it is not my place.
Even if his mother wants him to live in a bubble!