Our Neice Invited me to wedding but not my sister (her other aunt)

I agree with other posters. You should go because it's your niece but you shouldn't ask because everyone has their reasons for the people they do/don't invite to their wedding

Don't ask!
They have thought about what they want to do, extensively, and have acted upon it. There is no reason for you to get involved. Think back to how hard it was to plan your wedding, she does not need your input on this, I am sure she has spoken with her father on it and took a long time to come to a decision.
Of course you should still go. You said your sister was not invited to his other daughters wedding, but you did not mention your sister being upset about that or anything, and stated your niece is closer to you. While what she has done is usually taboo, there are circumstances for it. You stated your sister probably wouldn't even go, there is no reason to push this issue.

It's not your place to ask, and if she wasn't invited before to the other wedding I don't see what the difference is now, you stated that your niece is closer to you anyways. I wouldn't ask but I would attend, it's a niece you said your close to, it may damage your relationship if you don't. As a bride whose having to pick and choose herself, I can tell ya that my dad has two siblings, a brother and sister, I am 10x closer to my aunt than my uncle who I haven't seen in over a decade and has definitely burned some bridges with the family (even his own son who I am inviting), his wife is rude and obnoxious to my grandparents and I'm not dealing with that on my wedding day, I have too many people that I want there, to invite people like that, will I probably face some crap for doing so, yeah, but that's my business.

Who your niece has or hasn't invited is truly none of your business unless you're helping pay for the wedding.