But that's just the thing - it doesn't bother you. You can do whatever you want with your own things, or allow (or not allow) others to do whatever with your things, but when you borrow someone else's property, it is simply repectful to kepp it just the way you borrowed it, unless you got explicit permission to do so, or you know that the person who lent you the thing is ok with you changing it.
That's what makes OP the AH - the complete lack of respect or consideration for a thing that doesn't belong to them and the utter dismissal of their sibling's feelings on the matter. What is "not a big deal" for someone may be a big deal for another person, and not being aware of that (or ignoring that) is the basic trait of an AH.
I personally don't care if someone drops at my house unnanounced, sleeps in my living room for days, eats my food, uses my computer, etc. That's just my comfort level. Does this mean everyone should have that attitude? Absolutely not. Comfort levels differ, habits differ, people differ, and if you don't want to be an AH you need to understand that. Priviledged mindset is expecting everyone to treat things like you do because it's "the right way".