For these specific events, we already know there are only going to be three overlapping guests. THREE! They do not have to travel to get to Veritek’s wedding and will have months to find their outfits (the dad and grandfather can even re-wear their suits, so really, we are just talking about the mother of the groom here). If the parents are planning on a gift for both of their kids’ weddings, that would have to factor into their budget even if Veritek decides to get married this year. The SIL’s destination wedding is also incredibly small, and at a venue like Disney World, I doubt they have to do much more than pick their wedding package and show up. The SIL can have a shower/ bach party literally any other of the 7 weekends before her wedding- or even before that! It’s just not that big of a deal.
I would be more sympathetic toward the SIL here if the weddings in question had a different set of logistics, or if she hadn’t already made a thing about the couples getting engaged so close to one another OR if she hadn’t also gotten upset about Veritek leaving for her own honeymoon after the destination wedding. She’s clearly just pissed that she is not getting enough space to shine. I have three other siblings and would be nothing but happy for them if I were in this situation.