Sin is in the will.  Sounds like you went to appease your parents and did not will to go, so you did not sin.

Sounds like you're a Trad who knows the Novus Ordo is bad/dangerous.

Therefore you should avoid it. Period.

I think passive attendance at weddings/funerals for social reasons would be permitted, but no active participation.

Since you bring up Bishop Williamson, I have to clarify his position for you. He never said we (trads) could go to the Novus Ordo. That's not what he said at all. He wishes that everyone could have their eyes opened to the evils and dangers of the Novus Ordo, and leave their parish churches forever (until the Crisis is over) to attend only Traditional Mass venues thenceforth.

But that doesn't mean God will abandon every old Grandma who foolishly and ignorantly attends the Novus Ordo.

Look how many current Trads were Novus Ordo at one time. So that suggests God is writing straight with crooked lines, or something. There might be grace there -- perhaps just actual grace, but still some kind of grace(s) for those who honestly seek God and his will, but unfortunately are in ignorance through no fault of their own. I speak of actual graces to find the truth eventually.

But above all, we are not God, and we have to leave God room to be God.
Some things are a mystery. Remember, countless heresies (including the conciliar religion) spring from a desire to simplify or remove various mysteries. But as simple-minded creatures of limited intelligence, we NEED to deal with a certain amount of mystery if we're to grasp certain truths!

Not only can attending a Novus Ordo Mass be very damaging to your soul, but also it can scandalize others, your parents, and your friends.

Definitely this is an ask your priest question, so that you will know what to do next time this happens.

I have attended a Novus Ordo funeral with the permission of my priest as a sign of respect for the deceased, but I was asked not to participate in any way.

I think passive attendance at weddings/funerals for social reasons would be permitted, but no active participation.

This.  This is how the Catholic Church views attending non-Catholic services.  

The NO "mass" is worse than attending a protestant because of the element of sacrilege.  It is a sacrilege in that it either 1) mocks/imitates concecration, or 2) actually effects concecration and threats the Real Presence disrespectfully.

If you recognize the post V2 popes as legitimate, you recognize the Novus Ordo as legitimate by proxy because if they're true popes, they can't institute a heretical rite.

So, if you believe they are, then no. But if yes, maybe.

IMO, the fact that "we do not always" treat the Novus Ordo as another religion is part of the problem..and the reason why you (and others) do not know whether to attend it or not.

I personally do not want to go to the Novus Ordo, but I do not know if it is a sin or not to attend passively like I did with my parents. It seems to me that it might be a sin, but I respect the opinions of the SSPX priest I asked about it who told me it was not a sin, another SSPX priest who said it was okay to attend the Novus Ordo passively while not participating (not just for weddings and funerals), and of Bishop Williamson who despite Matthew's objection above, clearly believes it is not always a sin to go to the Novus Ordo because he said it was ok for the lady who asked him to go to the Novus Ordo. I could also add the opinion of Bishop Fellay who famously attended a Novus Ordo Mass in a convent and highly praised it publicly.