My niece had a wedding on a cruise ship. I went and had a great time. They are both very well off and I gave them a card.
I went to Las Vegas for another niece's wedding, besides my parents and my sister/bil, I was the only family member present. The groom's family - no one there. It had been planned for 12 months, plenty of time to plan to go. They were happy to have me there and did not expect a gift.
Now there were people who couldn't believe that I would go on the cruise wedding trip for a niece who is a step niece. To me no difference. Somehow the Las Vegas wedding made more sense to them, because she is my "full niece.