Both sisters are engaged, one is a bridezilla! Help

Here is the background... I was married ten years ago and my older sis (39) was engaged shortly after. Since she has left her husband (an amazing catch imo) because she’s no longer attracted to him and didn’t like their restrained budget... he’s an award winning lead copywriter/advertising specialist. Fast forward almost ten years later and my sweet little sis (28) is getting married, they were engaged in October. My OS has had nothing but terrible things to say about LS engagement including the dates, boring location, fiancé, her weight, it goes on. She still tries to set her up with her friends. So on New Years my OS got engaged. I found out she gave her boyfriend an ultimatum and that he was also told by his mother if he gets married they'll help buy a house for them. My sister’s ultimatum? leave him if he doesn’t pop the question by NYE. That day leading up to it was a whole other story. She tried to manipulate the whole day, her bf still hadn’t asked my dad’s approval the day of, he was taking too long so my sister took it upon herself to tell my dad, “hello haven’t you noticed ______ has been trying to ask you a certain question???” At that point I told her to try to let it happen organically in which she told me that I don’t get to Bleep up her day.. so now she’s engaged and she put on a great show for all our party guests mean while texting LS who wasn’t there how it’s not even a real ring, that’s she knew it was going to be a temp until they pick one out together but it was just a band. He ended up telling her not to take it back to Tiffany’s to just keep it o top of picking out the real one. Honestly he’s learned it’s easier to spend money than fight. The Next day she told everyone they’re getting married in Greece in September. This will be her second destination wedding. My little sis is getting married in Colorado in April and my OS just asked her to consider changing her date to October. Big sis ‘can’t make it’ to my lil sis engagement and bachelorette, she had previously okayed both dates with her. So now she’s saying her and her fiancé are going to take care of our tickets to Greece since she knows money’s an issue for us and literally told my LS I get it if you can’t make my wedding. How sad she feels so un important. It’s a generous offer but her attitude toward my other sis, the fact that most of the time she’s rude to me, the disrespect towards our mom and her own turbulent relationship make me feel like I can’t support her. They fight all the time, he’s a pot head which she’s okay with since he’s a super rich trust fund kid (actually he’s 41). The other issues are my lil sis is reconsidering their honeymoon to Switzerland because if she attends Greece wedding she’ll have to go back to Europe shortly after and honestly the way she’s making her feel she doesn’t want to support her either. I have a 7 year old who will be at the beginning of her school year in September. It makes me feel awful for my LS that she doesn’t feel like her OS cares if she’s there or not. I told my parents we need an intervention; her behavior is not okay & her relationship needs therapy. I told them I am not going to Greece esp. since my LS is heartbroken. My parents are afraid of her and said she’ll never forgive us if we don’t go or try to talk to her. Soo this leads me to the question, I know I’ll be putting my relationship with my OS on the line but this is not right to support. Thoughts?? Oh also she had these same behaviors 9 1/2 years ago when we were both engaged. She knows I’ve been planning my 10 year vow renewal in October which I’ve wanted to do at our family property in Mexico for years. I’m taking it off the calendar because i want to give my sisters time and when I brought it up she said, “you’re honestly still gonna do that?” In talking to her I’d like to bring up the idea of a local ceremony for our side of the fam. I know she’ll hold the money thing over us for ever but some of us don’t have rich bf. His whole family was already going to Greece in September so they’re planning it around that. Oh and they’re first trip to Greece he broke up with her the day before she flew out to meet him (he left 2 days earlier) and refused to pick her up at the airport in Crete; his mom picked her up. The next day after consoling her for hours she called and they were at lunch together and ‘they’re good now’. There’s just a lot.. I feel like I need to say something.