Just a couple things that come to mind, not much to add to good things already said.
When we are convicted of what is right, we can't let the world be our guide, our obedience due God, not man. In other words, we shouldn't fail to do right, because of pressure from those who are wrong.
That said, being only 18, and just my opinion, it's neither serving God to make this an issue any more than necessary in your family, if you're not completely independent of your family, maybe it's not wise to create the trauma of a break with your family, if, for instance, you still dwell with them, everyday, do anything that would turn your family environment to rancor. There is also something to be said for perhaps opportunities, while yet congregating with Catholics, to tactfully and wisely represent a Biblical view from God's word, be a gentle light in that environment. All Catholics are not unsaved, as some fools would say, rather many do have a true confession of the Lord Jesus we are not to condemn, many who don't approve of all things Vatican.
So, I think it's good we temper everything with love and peace, as much as possible, especially where family is concerned. If you're met with evil, so be it, but we are also called upon to try and win people by representing Christ in love, being Christ in other peoples' lives as best God gives us, be a light amidst a world that doesn't approve of us, not to hide from the world, put the candle under a bushel. That is to say, there's nothing wrong with being a light where God has placed you, even if your family were atheists.
We're not called upon to make war on people who disagree with us, unlike message boards, and your family doesn't cease to be your family, unless they disown you over truth, but that would not be in your power or your disgrace, then. Just some thoughts, having experienced some family dynamics.
1 Corinthians 9:20-22 And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law; To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law. To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some.
Though the thrust of this involves wives, I believe there's an equally valid principle of Christian behavior in the world at large, the validity of being that light we'd like others to also see in the word of God,
1 Peter 3:1-12 Likewise, ye wives, be in subjection to your own husbands; that, if any obey not the word, they also may without the word be won by the conversation of the wives; While they behold your chaste conversation coupled with fear. Whose adorning let it not be that outward adorning of plaiting the hair, and of wearing of gold, or of putting on of apparel; But let it be the hidden man of the heart, in that which is not corruptible, even the ornament of a meek and quiet spirit, which is in the sight of God of great price. For after this manner in the old time the holy women also, who trusted in God, adorned themselves, being in subjection unto their own husbands: Even as Sarah obeyed Abraham, calling him lord: whose daughters ye are, as long as ye do well, and are not afraid with any amazement. Likewise, ye husbands, dwell with them according to knowledge, giving honour unto the wife, as unto the weaker vessel, and as being heirs together of the grace of life; that your prayers be not hindered. Finally, be ye all of one mind, having compassion one of another, love as brethren, be pitiful, be courteous: Not rendering evil for evil, or railing for railing: but contrariwise blessing; knowing that ye are thereunto called, that ye should inherit a blessing. For he that will love life, and see good days, let him refrain his tongue from evil, and his lips that they speak no guile: Let him eschew evil, and do good; let him seek peace, and ensue it. For the eyes of the Lord are over the righteous, and his ears are open unto their prayers: but the face of the Lord is against them that do evil.
Whatever you do, patience is a Godly attribute, as opposed to constantly warring against those who may not share your faith.