I am unemployed right now, so I try to be mindful of my expenses and always account for my necessities first. My friend reached out and invited me to a private dinner party at her home and requested $200 for attendance. That would cover the cost of the private chef and some other things. I couldn't spend that kind of money on a party. I explained to her that I could not afford to be there, and since then, she's been cold.
My partner's sister is expecting her third child in December, it was quite a surprise, she found out at 20 weeks. I'm happy for her and excited about the baby, but concerned about the financial side. She and her husband haven't worked in nearly a decade. They're barely getting by on government support, and my partner has loaned her money over the years. She currently owes him over $1,000-though she pays small amounts back, she borrows more than she repays.
Let that person know that you appreciate being included and want to be able to go on the trip with them, but you do have limited resources. Ask what the expectation would be for expenses during the trip. This may feel awkward, but it's way better to learn now than to be away and unable to participate or contribute fully.
It's too bad that your brother and his wife didn't talk about these things beforehand and map out a plan for the future, especially something as important as where they want to live. Obviously, you cannot control what he does, but you might recommend to him that the two of them sit down and review their goals for the future based on available resources, needs and desires.