Îi is for plural masculine, le is for plural feminine.

Why don't you (have to) use them in English?

Your sentence is grammatically right, there is nothing wrong with it, however it sounds odd when you talk about known, well-defined/well-delimited/well-specified etc things (like "your" sisters, and generally, "articulated" stuff, from the grammar perspective, things that would need to use "the" in English). If you hear some sisters (like girls or women in general, or persons members of some "sorority", like Bene Geserit :D), then you could say "I hear sisters", or "I here the sisters", which would be respectively "Aud surori!" or "Aud surorile!" (if it is known or not who they are). Like you put a cup on the wall and put your ear to it, and have the revelation that you are hearing women talking in the next room, you would say "Le aud!", or "Aud surorile!". But even so, it sounds a bit odd. The correct way that goes everywhere is "Le aud pe surorile voastre". Without the stressed words it sounds odd.