How can I unlock my iPhone?

I got an iPhone from my sister but I can't log into my iCloud because the previous owner still has his logged in. I can't sign out in any way and I'm afraid that if I reset the phone completely, it will still ask for his iCloud.

You NEED the original owner's Apple ID. There's NO way to bypass it.

If your sister was the previous owner, ask her to remove the iPhone from her iCloud account at icloud.com/find

If she is NOT the original owner and has no way to contact the original owner, then you are out of luck.

You need to have the owner unlock it. That's one of the disadvantages of a stolen phone. (Another disadvantage is that he knows where you are as long as the phone is locked to his account.)

The owner will need to remove the phone and if he can't remember it then he needs to talk to apple and get it all figured out to reset the password. Apple can remove it assuming hes the original owner and kept the paperwork and proof of purchase or so i've heard but you need to have the paperwork and sufficient proof saying that it isn't stolen.