Some verses to help your family understand why they believe the way they do.
Romans 8:16 The Spirit itself beareth witness with our spirit, that we are the children of God
John 14:17 - Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you. Here's some more, Jhn 15:26, Jhn 16:13, 1 Jo 4:6.
1 Jo 5:10 - He that believeth on the Son of God hath the witness in himself
Romans 8:9 - But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
Why some don't understand this.
1 Cor 2:14 - But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.