Settle A Debate About Being Authentic

My husband and I have been talking about my sister. She tells me all the details of her dating life as she dates guys, really starts to like them, gets super upset and emotionally hurt by them, and then repeatedly forgives them and goes back to pursuing a relationship with them. This is a pattern for her so I’m pretty used to it and it doesn’t bother me. I respond pretty neutrally to both the lows and the highs, only give my opinion if asked, but mainly I just make vague comments and try to be supportive. For example, when she told me about getting back together with someone, I just said “Oh wow, I don’t know what to even say” and then said “But I’m happy you’re happy again” (Just to give you an example of how I respond.)

So back to the debate.. My husband and I agree that some of her behavior is stalkerish and crazy. We agree that so and so is no good for her. We agree that a relationship built on that foundation is never going to work out. SO is it unauthentic of me to not say those things to her? Am I “letting” her act like this and not telling her the truth (in my opinion)? Or is it authentic to stay out of it and just not lie about how I feel?