Notice the behavior of the OP.
This behavior is common amongst Judaizers..the accusation that others are in sin, because they define sin as non-observance of Torah.
If this is true, then they need to obey all of Torah, including going to Jerusalem three times a year for the pilgrimage festivals, making the appropriate animal sacrifices, and giving their tithes to the Levitical priesthood, as well as not touching a menstruous woman.
The Mosaic Covenant was one unit, and modifying any commandment isn't acceptable.
Their answer will be, but..we can't go to the Temple to have animal sacrifices performed, and the Levitical priesthood isn't around to give tithes to...what does that tell you? It is impossible to fulfill the Mosaic Covenant. Your modified, pick and choose list of commandments that you call the Mosaic Covenant is NOT the Mosaic Covenant..it is your own stripped-down version of it, and it is an affront to God. It is like holding a blade of grass up in front of your groin to hide your spiritual nakedness...you may think you're hiding your spiritual nakedness but God sees your nakedness clearly.
Additionally, why do you think Sabbath-breaking or eating unclean meats is NEVER mentioned in the epistles of Paul as a sin? Why do you think that John called the festivals the "feasts of the Jews", implying that they are not applicable to Christians? Do you realize that New Testament Christians, in many cases, were Gentiles who were enslaved to unbelieving Gentile masters who would not let them observe Sabbath or festivals? In light of this, why don't we read ANYTHING from Paul concerning the need to keep festivals and Sabbath regardless of this constraint?
These are all questions I had to ask myself as a former Judaizer to see the absurdity of those who taught me.
Regarding the definition of sin, I would define it as the moral requirements of God applicable to me, not the ancient nation of Israel, which was Torah. We know from Galatians that Torah was temporary, therefore Torah doesn't define sin for me.
Intuitively, the Holy Spirit guides the believer into realizing the moral requirements of God and fleshly behavior. Through regeneration, God writes his laws on our hearts, and that is how we know what is right and wrong. That is why Paul, in the book of Galatians, focuses on the theme of being led by the Spirit in Galatians 5. That section is not there in a discussion of the New Covenant versus the Mosaic Covenant for no reason.
Anyways, consider your inconsistencies. I doubt very much that you go to Jerusalem 3 times a year as the Law required. I doubt you build a Sukkoth and occupy it for the 7-8 days of the Feast of Tabernacles either. I doubt that you never touch a menstruous woman..unless all women in your life wear a T shirt on the days they are menstruous or unclean during the specified period.
And..I know the explanation some give claiming that this only applied to the priests..bogus explanation. There's no contextual proof in this regard.