But Then,Teacher, WHO Is My Brother?

Borrowing from history~~the picture above is of the Hovelson brothers, circa 1910 or thereabouts. Not a real car, though they were truly brothers.
Heard a sermon today about "Falsity In The Family"~the Family, of course, being the Family of God.
Conundrum Corner~if some are false, are they actually, truly IN the Family?
Jesus Christ, Paul, Peter, Jude and others all warned against 'false brethren'.
No where in Scripture can I find it stated that false believers will be with the Family of God in Heaven.
So...we pray that that God's Will be done on earth as it is in Heaven.
Would you consider it God's Will on earth that we welcome false brethren and teachers into the Lord's Church?
Or, as is stated throughout Scripture, are we to be wary of such false believers?
Even, perhaps...confronting them, and NOT fellow shipping with them?
Paul stated to the Galatian church: 
"⁶I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 
⁷which is really not another; only there are some who are disturbing you and want to distort the gospel of Christ. 
⁸But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 
⁹As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!"
Here's the thing-there are dead churches full of unsaved false believers which have oodles of theology promoting a different Gospel. 
Paul doesn't say, "Y'know, there are some true Christians in those churches, so we should fellowship with them even though they have a wrong Gospel."
Some may say, "You can't play God, Doug! You don't know who is or isn't saved!"
No, but it's not about me. I follow the God who said,
" ²¹“Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. 
 ²²“Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ 
²³“And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.' "