How can I help people who believe in spirits?

You ask how you can change your family whose faith is dependant on ancestral spirits, sangomas etc.

 One thing we should all be aware of is that we cannot change people ourselves. It is only the Holy Spirit of God who can change people inside, but we can do something. We can pray. God has chosen to let us be co-workers with Him by prayer. He works in answer to prayer, but sometimes we need to grow in our Christian lives so that the way we pray is more dependant on God. Sometimes as we pray, God needs to change us and make us more loving, more honest and trustworthy so that our families can see that being a Christian is better than their way of life.

 So that is our answer: Pray - that God will work in their hearts and also make us better witnesses for Him.