It seems to me that Jesus taught the best sermon was the one that is lived, not spoken. The world and many within "the church" see Christianity as a "RELIGION", and although you can look at it from that perspective, the "real" Christianity is a life-style that is rooted in a personal relationship with Jesus Christ; not adherence to a code of laws as was the case in the Old Testament. The best way to witness is to live what you believe, and be familiar enough with God's Word so that you can show anyone who asks about the "difference" that they see in you. It is not enough to simply tell someone what you believe based upon what you have heard someone else say; a Sunday School teacher, a Pastor, and Evangelist, a friend, etc. I dare say that most church members would be hard pressed to provide Scripture references for their beliefs, let alone be able to explain the plan of salvation or other basic doctrines of the Christian faith.
To compound the problem there are the differences in denominations that all claim the Bible to be the inspired Word of God and their basis for faith and practice (or at least part of the basis for their faith and practice), yet their interpretations vary widely, even within a denomination. No one in their own power can "tell" anyone else what the Word of God says, nor does anyone have a mandate from God to do so. It is our job to be prayed and studied up such that we can be empowered by the Holy Spirit in all humility to be the "light" and testimony for Jesus Christ, especially within our own families; because that is the hardest place to maintain our testimony; with those who see us, not at our "Sunday" best, but as we truly are: sinners saved by grace.
Pray for your family members. Read and study God's Word so that you will be equipped to explain not only what the Bible says, but what it doesn't say, which is sometimes far more important. Make sure that your life is right with God and that your "light" is shining brightly. And most of all, wait upon the Lord to provide you with the right time and place to share your faith, your beliefs, and your understandings; always ready, willing, and able to listen to the questions and objections that someone else may raise about your faith, the Bible, Christianity, etc. There are plenty of people out there who have been misinformed about what it truly means to be "Christian"; about what the Bible says and teaches; about the relationship of Christianity to other world religions. God tells us in His Word to develop the Christian characteristics of love, humility, patience, mercy, etc., the very characteristics of Jesus' life. His Holy Spirit will take what feeble attempts we make as a testimony and witness, and honor them with the fruit that sometimes we are all too often so impatient to harvest.