Dressed or not, it's your brother-in-law. He is your sister's, so you have nothing to feel guilty about. You didn't steal anything from them, you didn't owe them anything, they didn't take anything from you, and they didn't owe you anything.

In this case, you should pretend that nothing happened and live as usual.

Everything is the same.

Don't tell your sister that you just had a dream. You just saw the naked adult of the opposite sex. Where can't you see the naked body of the opposite sex in this world? On TV, in calligraphy and painting, on the Internet, and so on. Naked, regardless of gender, there are not many secrets and mysteries, don't take it seriously.