<p style=”text-align: left;”>This text may sound hard, as I got swept away in empathy for your sister. I’m sorry if my words are hurtful that’s not my intention. I want to show you another perspective on the narrative so you can be able to fix the relationship with your sister. I really think you are not taking enough responsibility in this topic.
As a middle sister myself, I would feel deeply hurt and rejected, betrayed in that situation. If my sister would talk about probably not being able to go on two trips in the same year, I would not make sistertrip-plans until she said okay, she has a budget available. I wouldn’t want to push my sister into debt for a trip with me. And then she books a second trip without me and without even asking me? Ugh!

How to proceed? You made a sister-trip in 2023 impossible by booking the last min. Period. Fix that.
Cancel the last min or make new plans for a big trip with your sisters for next year and make a booking so there’s no room for doubt.
Yeah, she should have communicated better, same counts for you. You booked the second trip while she was still thinking you couldn’t afford it. Choose her first, then she will be able to show that kind of vulnerability you want from her and take part of the blame for not talking to you about this topic. And yes, what she did last year was not nice either, but you talked it out and you made up, so you can’t pull that back into the discussion, that’s not fair, that’s making her the bad guy and again avoiding your own responsibility.
Talk to her, check if she was indeed waiting for you to say you can afford the trip, ask how she wants you to fix this and choose her first. After all, she is your best friend.</p>
Invest time in your sisters (counts for all three of you) and plan more smaller things together, it’ll make future mistakes like these less heavy.