But people’s ideas of happiness come from familiarity with the world, not familiarity with the joy of heaven.
Our typical speculations—including speculation about family members being in heaven or hell—is based on worldly standards. These standards are necessarily derived from what we experience in this world, in the absence of any direct knowledge of heaven or hell. Therefore, whether we affirm the existence of God or deny it, our belief is based on faith.
If God is real, then he is a being of perfect justice, love and mercy. We can’t know these attributes of God until we are in his presence in heaven.
It would be silly to expect his creation, under the curse of a fallen world and the deception of Satan, to understand, intuit or to feel what the reality of God’s love, justice and mercy is like.
If the Bible is true, then everything you see and experience is evidence for God. The only way to conclude otherwise is first to choose atheism and then to rationalize that there’s “no evidence.”
If you don’t care about the eternal suffering of your non-Christian family members once you’re in heaven, why would you care about their eternal souls in this life?