Yes.Your brother's brother-in-law is the brother of your brother's wife and is not related to you.Of course, your brother's brother-in-law may also be the husband of your brother's sister. If you are...
If he's not married and there's no blood relationship, you can if you want, and if he agrees. If he is single and you also is free and you there is love binding between you then go on.
Yes, unless they are close biological relatives like brother and sister etc. They are only in your family because you and your spouse got married. That does not make them related in any other way.