“Brotherhood is a two-way street.” -Malcom X
Treat others as you would wish to be treated. And when they fail, forgive and give them a second chance because there is nothing more American (or human) than forgiving your brother and assisting them in their recovery from a downfall.
But if they are unwilling to help themselves, at some point you must stop caring about them more than they do because like the drowning man they will drag you down and then you can save no one else.
If they continue to act with duplicity and malice, they violate the covenant of brotherhood and can no longer be trusted. Honest mistakes, screw ups, or unforeseeable events are all acceptable and forgivable but jealousy and ill intent are not. Repentance is required for true forgiveness and ongoing assistance.
Trust your brother, until they prove they are no longer worthy thereof.