TL;DR: How do I convince my sister that taking communion without confession is a big NO NO
Recently, my sister came with me to mass. I was quite happy about it. We were raised catholic… kind of. My parents, in their youth, were in an almost sectarian group led by a liberation theology priest. I could rant about how heretical it all was, but suffice to say the priest left the priesthood to pursue a relationship with one of the teenagers of the group (yikes). So me and my sister were raised with an approach to religion on the lines of "this is a nice thing to give some structure and moral values to your life until you grow older".
Fortunately, we always were bothered to find the meaning of life, to find some purpose. My sister has always been more carefree and more free-form. She once started attending mass, only to stop cause she didn't like the priest much. I've gone through some rough times, and eventually came to the realization that I needed radical change in my life, I needed to direct it towards something greater than myself. So, I returned to the faith (thanks in no small part to this board), started with daily prayer and lots of Fulton Sheen videos, then talked to the priest, did my first confession in 10+ years and now I'm working towards my confirmation.
So, talking one day with my sister, I offered her to come to mass, assuring her that the current priests are actually quite good. She came, to my delight, but to my horror, I saw her stand up at the end of the mass and join the communion line. I stayed seated and didn't say anything. I hoped she was taking it as a way to rejoin or something like that. Maybe she even had confessed, and I didn't want to be judgmental. Next Sunday, our parish's mass was aired on TV (and, God know why, instead of the organ that always sounds, the guitar was played). She took communion again, and I remained seated.
Later, back at home, my mother said she had been watching on TV. She stated that she saw my sister in the communion line, but not me. I answered "I would never take communion while in a state of sin. That shouldn't be done". My mother replied, a bit mockingly, "Oh, and what sins do you have?". My sister, visibly offended my my answer (though my comment was not directed to her) said "That's not how it is, I don't agree.", left and went to her room.
How do I get her to understand what the Eucharist really is? She doesn't even seem to fully understand what sin is, and has a mindset of "I'm a good person and therefore I don't sin". I don't want to go full "REPENT!" on her and turn her away, but at the same time I fear that such a banalization of the Holy Sacrament can completely destroy the faith she's finding. She's smart, and will understand complex ideas if they're presented to her, but she's also prideful, and if I confront her directly with "this is wrong and this is why" she'll reject it frontally, and may even stop coming to mass.
How would you act in this situation? Have you dealt with anything similar before? Any help is welcome.