zeropointone

Would you like to give some examples?

Short-term homelessness (2-3 months) seems to be tolerable for most housed relatives if the homeless person can make up for it later. But after 6-12 months, the housed will all be gone no matter how close they were with the homeless because the poor individual is marked as lost. The lost status remains even if the homeless person makes it out much later. It's very similar to health conditions like cancer or depression. If you go over a certain threshold, it's over.

As much as you're not "allowed" to run a marathon without limbs. As much as you're not "allowed" to pilot an aircraft as a blind person.

I heard that "If my brother/sister/mother/father" story plenty of times. And it was never true. I've seen and heard too much while I volunteered in homeless shelters.

They want to continue living their lifestyle, just like OP. That's not necessarily selfish or entitled. It's just two incompatible ways of life. The solution is to cut ties and find people who are compatible, for both OP and his sister and her husband. Walking away in peace is always better than endless conflicts.

If they won't accept that then OP can't do anything else than moving on. I had to do that too with friends and relatives from higher socioeconomic classes. It's not possible to make people understand something they are unable or unwilling to understand. It's not possible to force people to have something in common with you. And that's okay. That's life.