Yeah, I can understand the impulse to get the audience thinking, hey, wait a minute, Aredhel is still single....she could get married. I just don't think that her father or brothers should be discussing that hypothetical situation in the absence of any eligible young man in her life. And they certainly can't 'prepare' for Eöl's marriage-by-kidnap. When Finrod and Galadriel discuss Celeborn, it's because Galadriel is clearly in love with Celeborn, and their marriage is a distinct possibility. If we do want it to come up, it might be when Curufin is dismissive of the news of Galadriel's engagement. I don't think we want him to say that if any of his cousins were going to do something dumb and flighty like that, he'd expect it to be Aredhel, not Galadriel - that's just completely tactless. But we could somehow include Aredhel in that discussion. Maybe she could express, I don't get it either, but Angrod assures me that they are happy together, so who am I to say otherwise?
I have had conversations with very close friends and family members about their significant others, and sometimes rather serious or heavy conversations which are very much a 'hey, I don't think you should marry this guy.' But you have to be really delicate when you do that, because that's not something you can tell someone else to do. It's one thing if you're the parent of a teenager - you can set rules and restrictions on contact between the young people, and hope to deter unwanted relationships that way (although...good luck). It's quite another thing if the people involved are adults. Then, you have to couch it in terms of 'you don't seem very happy' or 'you haven't been yourself lately' or 'hey, is something up with you two?' - inviting the other person to articulate whatever the issues are, rather than telling them that there's issues. And if you aren't really close good friends/family...you really have no business initiating that conversation (almost) regardless of circumstances. [The only exception I can think of is if you know a 'secret' that you feel obligated to share with the person's significant other...and even then.]
And...at the end of the day...no matter how close I've been to someone, it has *never* been up to me whether or not they got married. Cause...that's a decision you make yourself. Hopefully with the love and support of both families, but...that's not actually a requirement.