Halachically speaking, if you are in shul and don’t have tefillin for whatever reason, and no one is available to lend you a pair for whatever reason, it is mutar to use a pair that you find, provided you handle them gently and put them back exactly as you found them. This, for once, does not come from my husband– I just read it in an ArtScroll book about the halachos of traveling.
Regarding a family home, in my house we ask if the person is around and just take if he or she isn’t, unless it is something that we know is precious or irreplaceable, or something not hygienic to share. My brother and my husband wear similar size shirts, so my mother gave two of my brother’s shirts to my husband when he realized he hadn’t packed enough. And when I’m not home, my mother frequently borrows my clothing (we are also similar sizes). Usually she’ll just let me know the next time we speak on the phone (“oh, by the way, I borrowed your denim skirt, I hope that’s okay”) and it’s fine. But I think it really depends on the family dynamic and how comfortable people are. In my opinion, if you don’t know or can’t guess how your family members would feel about you borrowing their things, you probably need to work on your family relationships.