Re: Siblings that are possibly "involved" with each other!
If neither of them are married, it is cheaper for them to live with someone. If they get along, then why shouldn't they live together? Or do you think it would be better if they live with a stranger? I've known people who hated their housemates, whether same sex, or opposite sex. Living with a family member you get along with is much better than living with a stranger you may or may not get along with.
In many foreign countries, family relationships are very close and they look after each other. Emotional bonds do not equal sexual contact.
As far as going on "romantic" vacations together, I don't know what you mean. Friends go to places together where honeymooners also go. Doesn't mean the friends have anything but friendship going on. It is better than going alone.
You are making a lot of assumptions based on your own perceptions. Maybe your perceptions and assumptions are wrong?
Have you seen them having sexual contact of any sort? If not, you are being very unfair.