Answer
Start with Frank Sheed's Theology and Sanity, which contains a lucid and compact explanations of this subject. Then loan your relatives the book and offer to discuss it with them after they've read it. During the discussion cite these passages as biblical evidence there is only one true God: Isaiah 44:6; 45:5-6, 18, 21-22; 46:9. Cite these passages to show God is omnipresent: Psalm 138 (139):7-8; Wisdom 1:7; Jeremiah 23:24; Ephesians 1:23.
