The cousin who lent you a suit for your first interview did more than lend a suit.

Walking into a room dressed correctly is half of being able to speak in it. The suit itself was borrowed cloth; what came with it was the feeling of not being out of place, which is what lets you answer questions like a person rather than an apology. Somebody in your family had the thing and handed it over without making you ask twice.

Return it clean, which is the whole etiquette, and tell them how the interview went whether or not you got it. Keep a note of who in your family has what, and offer your own things before anyone has to ask.