A few weeks ago, DH's cousin announced that her & her husband are having a second wedding.
Their first wedding was very quiet, immediately family only (so we weren't invited), and we found out about it after the fact. No problem with this at all. I'm a firm believer in doing what you want for your wedding. We weren't offended not to be invited as we are not really close to that part of the family, but were happy to hear the news and sent a card and a bottle of champagne.
The second wedding announcement was on Facebook with pictures from an engagement photoshoot and said something like "we're planning a big party, you all better be there!". Along with about 200 other people, both DH & I liked the status and sent texts saying that the party sounded fun and we're looking forward to it. I was a bit 
at the engagement photoshoot but thought each to their own, maybe they didn't get the ring / wedding they really wanted first time around and are making up for it now.
DH is away for work this week. Last night, DH's aunt (mother of the 'bride') rang me. Aunt starts by telling me how upset they are that we haven't sent an "engagement present" to the cousin yet. I laughed and said "but they are already married!" ( also thinking, regardless of whether we were to get them a second wedding gift whenever it happens, SURELY people don't give "second engagement" gifts to married people?)
DH's aunt has taken grievous offence at this and proceeded to tell me that it is unfair that we got "thousands and thousands of pounds worth of wedding presents" and her DD missed out. And that I "needn't think that bottle of champagne lets me off the hook" and that we're expected to get her DD a "proper wedding present".
She started to gear up to complain about various things about our wedding (which was nearly 5 years ago!) but I was having none of it and told her that I wasn't going to discuss this any further, we're not sending an "engagement present", we'll decide on anything further once a second wedding invitation actually materializes, and that if she wants to slag off our wedding she can ring someone else.
I've spoken to DH and told him to deal with his rude aunt when he's back from his trip. He was totally shocked at her cheek but mostly we had a laugh about it.
I've since spoken to my lovely MIL who has also had a call from the aunt along similar lines. Aunt & cousin don't think its fair that she "missed out" on having her day being the center of attention. She specifically brought up the amount of money her DD received as wedding presents vs what she thinks other family members received for their weddings.
I'm not being unreasonable am I? You don't just decide to have another wedding because you want to fleece your relatives? And even if you do have another wedding, you don't get flipping engagement presents? And if you want to tell someone the gift they got you was crap, you don't wait 3 years to do it! 
Also, just for context : DH's cousin is the same age as us (and we are all in our 30s!) We said "no gifts" for our wedding because most of guests were travelling a really long way. However, some people did choose to give us gifts anyway and we were really grateful for these.
The cousin didn't get us anything, and the aunt was very offended that we had said "no gifts" because this was apparently was a clever rouse to make people give us "thousands and thousands in cash" instead of presents. The aunt gave us a hideous fruit bowl about six months later and said "I don't agree with subsidizing people's honeymoons" when she handed it over