My sister's secret relationship and our family's standards

As-salamu alaykum. We come from a practicing Muslim background-my parents are even Hafiz of the Qur’an-but, honestly, our level of faith varies among us. We might appear like a strong religious family from the outside, but inside, it's not always like that. My oldest sister has been secretly talking to a man for several years-they’ve been meeting and calling without our family's knowledge. He only recently came to ask my parents for marriage. What bothers me most isn’t just that the relationship was haram; it’s the huge disrespect he showed our family by going behind our backs for so long, building a bond with my sister, and sharing affection privately. To me, that shows a serious lack of adab and taqwa. Even during Ramadan, their behavior didn't change much. I know my sister also has shortcomings in her practice and is partly responsible. But that’s exactly my concern: just because she’s struggling, should we lower our standards for who she marries? For me, it's about our family’s values-do we accept someone with weak religious commitment and character into our home, or do we stick to our principles no matter what? As the eldest, her marriage and children will influence our younger siblings’ futures, which worries me that our family might drift toward cultural norms instead of sticking to deen. I’ve advised her to end this haram relationship; she admits it’s wrong, but hasn't stopped. My parents haven't really looked into his character deeply yet. So I'm stuck-should I speak up and try to discourage this, or just stay neutral? May Allah guide us.