Some don’t believe at all, and they can be the toughest to reach. The freedom argument will likely help; the fact they must let you have freedom to believe, because of the country you live in. So could some of the items mentioned in the first section of this book. Ask them to talk about their relationship with their father, if they can. Our Heavenly Father is so much greater than any man.
Also, the field of apologetics is an important one to study. That doesn’t mean how to apologize. It just means making arguments outside of the Bible. A couple very good ones are the series by Lee Strobel, such as “The Case for Christ,” and Josh McDowell’s much larger work, “Evidence That Demands a Verdict.” Perhaps you can convince a loved one to order one of these. Each man was an atheist who started to write trying to disprove God, and wound up coming to faith in Christ instead.
We don’t know whether the Shunammite woman’s husband worshipped idols, or didn’t worship anything but the “good life,” like so many in America have over the last few decades. It was probably the latter; the Bible usually says if a person worshipped idols. Her husband would be an example of someone who likely didn’t seem to care one way or the other whether his wife worshipped God.
However, others are much more vehemently opposed. Some will simply make snide remarks. Others will be more hostile. Except in the most extreme of places, you should be able to shepherd your younger siblings, even with an unbeliever as a parent. Just remember to live a life pleasing to God, and show them that you love them.