I agree with specialmom, but I will say I can't follow this story at all.
It's clear that your now husband has acted in the role of daddy to her, is not her daddy, and didn't know her until she was already born.
I can't tell who the other two are - one was a fiance and one was a friend, I guess, although it seems you aren't completely sure who the dad is among those two, although your niece/cousin seems to know somehow.
I'd say don't tell. Rarely to children in stable loving homes "hate" someone for glossing over an ugly truth and choosing to give her an intact family.
Hi there. Well first, your husband IS her father. He acts as her dad, is raising her, loves her. This should breed security on her part. And on your part and his part.
This younger cousin is rude and inconsiderate. What an angry person to make a big deal out of this. It says a lot about this cousin and their low character.
She's 8. My kids haven't had the sex talk yet at 8. They don't understand that there is a bio dad that just gave sperm to make a baby and then there is someone that acts as her father and IS her father of the heart. In a few years, tell her.
And I'd tell that cousin that you will eventually tell her but until you and your husband decide to do so that she needs to be quiet. It's none of her business and there is a screw lose of someone that thinks they should involve themselves in such family matters.
good luck