لحظة is somewhat more formal, because استنى is an imperative and can come off a bit too imposing. (As a side note, both استنى and شوي are colloquial and not MSA).

Another thing to keep in mind is that in Arab culture, you show respect by using a title to address people. You use أختي (sister) for a woman around your age, أخوي (brother) for a man around your age, خالتي (aunt) for a woman older than you, and عمي (uncle) for a man older than you.

So you might say: لحظة، أختي