I guess that most of us know, how serious problem it is. If not, just look at Luke 9:26. Now, I have a problem. I don’t know if I am one of cowards of Revelation 21:8. First: I have openly said to a friend, that I’m a Christian, knowing that there is a possibility that the reaction may not be nice. And while it was exactly opposite - kind and good one, and you could say that I’m not ashamed of Christ, I have still felt a slight discomfort. Not enough to silence me, but still. Second: my parents are, or at least they behave like they were, “Christians, but only by name” - they consider themselves Christians, but act basically in such a manner, that it would be hard to discern them from non-theists. I’d like to talk about it on occasion of Sunday, before going to church - I just feel that it would start conversation easier than out of nowhere. Is putting a task few days in future, to avoid awkward start of the talk, being ashamed and therefore sin?
You sound like you are falling into two very serious errors here (not sins–not yet). Firstly you are letting scrupulosity take over your thinking. And secondly, you are judging others (in this case your parents) hearts and minds as if you were their judge/have the right to judge. Unless they are actively involved in living a life of grave sin it is not your place to tell they they aren’t living their Catholic faith as well as you think they should be.
Judging others and scrupulosity, when not caused by mental illness, are two signs of developing pride in your soul. Don’t let that happen. No one is free of sin, but most people who are people of prayer are not deliberately sinning. You know you aren’t, so why fall for a lie that says you are?
And as to judging others behavior/lack of spirituality, we all need to look to ourselves first. As I wrote above, unless others are committing grave sins the state of their souls is not for you to judge. Pray for them and be a good example and leave them to God. At this stage in your life you are not mature enough to be telling your parents how they ought be living their lives.
Do not let yourself think you are sinning at every moment in every little decision you make for that is a form of pride that says you are worse then others when in fact you are no worse and no better than most other faithful people.
If you are waiting for just the right moment to discuss this with your parents, then I don’t know why you are worried about sin or shame. If you are putting off speaking with your parents because you are ashamed to mention the topic, then that is different – that would be sinful. God bless you.