I disagree with giving her a bible or rosary bracelet, those things are our pearls, they are very holy things. We are told by Our Lord to not cast our pearls before swine or give that which is holy to dogs. And I do not think it's a good sign you were even invited unless they invited you planning that you would not attend anyway. 

Whether one considers it a sin or not, prudence dictates the OP not go. After all, what exactly are you celebrating if not a NO ceremony? You can fool yourself and say "well, I'm going to, in some way, try to be an instrument of conversion for my niece or family." If that's the case then bring a podium with you and get to preaching.

Otherwise, know that by going you are celebrating a NO ceremony.

Yes I was thinking along these lines. Since the party represents the ceremony....

As a side note (I know you're not invited to the ceremony): I'm not sure if it would be sinful or not to attend the actual ceremony because I was always under the impression that passive attendance was allowed for non-Catholic weddings and funerals (not specific religious ceremonies in the non-Catholic religion).

I disagree with this. Valid Marriage is between 2 baptism Catholics. Everything else is fornication.

I disagree with this. Valid Marriage is between 2 baptism Catholics. Everything else is fornication.

Wrong. 

That’s your niece.  Traditional Catholicism isn’t a rigid occult.  Bringing her a gift is to encourage her in her Catholic Faith.   It’s out of charity.  Your wacky actions are going to turn off many from seeking Traditional Catholicism.  

Do you celebrate birthdays?  After all, they were baptized novus Ordo. 

This is your niece. If you don’t go to the party, it will just upset your family which will be lack of charity. 

Upsetting someone doesn't mean you lack charity. Truth is the measure of charity.

Caritas in veritatis.

The most important question hasn't been asked, which is (something like): What does this confirmation mean to your niece and her family? Are they under the impression that the girl is being confirmed as a soldier for Christ, growing in grace and being fortified in her resolve to stand firm against the enemies of religion?
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If they are, then setting aside the invalidity of the sacrament, they have good Catholic impulses which should be affirmed and can be affirmed without giving scandal. Attending the reception seems like a good idea at least.  You can use mental reservations to avoid agreeing with the invalidity of the sacrament and of the Conciliar Institution. 
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If, on the other hand (and probably more likely, unfortunately) this occasion is viewed as just some vague, cultural rite of passage, the only real reason to attend the reception would be to be a positive influence to correct that. If you have a good relationship with the people in question then you can use that and talk to them about the true meaning of the sacrament of confirmation. You don't have to bullrush in there and grab the microphone to announce its invalidity. But you can tell them the traditional doctrine, see how they react, see if it piques their interest.
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There is also a middle ground between the two-- that the niece and family do have some vague understanding that confirmation is an important religious ritual, to be done for its own sake rather than just for the maintenance of culture/custom, but are very ignorant of what it actually means. In that situation you could justify going to hopefully find yourself in a position to instruct or edify by sharing the traditional doctrine.
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This occasion may be one for fraternal correction, but bear in mind that fraternal correction is something that is governed by prudence. We do not have an obligation to compulsively and severely correct all errors at all times; on the contrary, sometimes we have an obligation to refrain from fraternal correction. One of the factors St. Thomas requires, when discussing whether we should correct others, is if our correction can be predicted to do any good. And when issuing fraternal correction, our goal is to succeed in converting hearts. So don't just go there and start flapping your lips about Novus Ordo this and Novus Ordo that, as though it were a box you needed to check. That would be rash, and would probably jeopardize future opportunities to positively influence these people.

"For do I now persuade men, or God? Or do I seek to please men? If I yet pleased men, I should not be the servant of Christ."

I am not concerned with displeasing them if I don't go. I am just unsure if going would be implying that I support a doubtful sacrament, personally I don't believe it's (novus ordo ordination) valid.

She's not going to  the service, she's just going to the party. This is a no-brainer

I can't believe there are people who are suggesting she/he take this opportunity to teach (not to mention the subjective advice like well, if they think this way then....but if they think that way, then....).  No, this is not the time to "teach".  And this is not something that we should treat subjectively.  

OP:  If you feel strongly that attending this event would effectively mean celebrating a lie/false religion, then just don't go.  Will it be easy to do that?  Of course not.  But that doesn't mean you should go.