Okay, headcanons…this time are purely headcanons. I often show an excerpt from a source to justify the existence of the headcanon, but this time the source is my feelings and that's it.
Headconing Cassandra and Helenus are fond of snakes and ravens because of the association of these animals with Apollo. So you might hear Helenus and Cassandra talking in an affectionate tone of voice about how something is "so beautiful" and you think it might be a baby horse, considering this is Troy we're talking about, but in fact Cassandra has a snake around her neck and Helenus is cooing at it. You hear Cassandra in the temple of Apollo singing softly and some bird chirping nearby and you think it will be a Disney Princess scene with blue birds, white doves and things like that, but in fact it is a raven. Cassandra isn't good with horses, she doesn't even feed them as calmly as Andromache. But snakes and ravens? That's what she's here for. Hector tried to teach her how to have a better relationship with horses, but failed. He consoles himself with the thought that at least Helenus is better with horses than her, although this is mainly because Helenus had to learn to prepare himself to be on the battlefield even though he prefers the part of being an advisor to being a warrior.
Considering Pseudo-Apollodorus' version where Deidamia marries Helenus, I'm now imagining Helenus randomly showing up with a snake wrapped around him all excited to show it off to his new wife as an attempt at connection since it's an arranged marriage and all (Neoptolemus gave Deidamia in marriage to Helenus) and Deidamia looking on in horror as Helenus continues to babble about the pattern on its scales. Helenus asking Deidamia if she thinks Lord Apollo would be happy if he found snakes for him and Deidamia is like "I don't know. You know more about that kind of thing than I do" while trying to keep a very serious face and trying not to look scared. Deidamia kind of remembering the time Achilles, still Phyrra at the time, tried to convince her that snakes were cute animals and thinking to herself why her lovers are like that (nobody asked, but this is a reference to the fact that Achilles had a cult on the Black Sea that actually included snakes). Helenus has more luck introducing her to ravens. She thinks they're intelligent and so they're really the kind of bird Helenus would like.
And when Andromache marries Helenus after Neoptolemus' death, she kind of already knew about this trait of his because they were brothers-in-law before and is already used to it. She even occasionally praises the snakes and ravens, although she thinks ravens have an unnervingly observant look. But well, she was there when the news broke that Neoptolemus died after displeasing Apollo, Andromache isn't going to be the one to drive the unnerving raven out of her bedroom window because who knows the risks of that. What if he really is Apollo observing Helenus' life or something? There's no way she can know! And she actually becomes convinced that the animal is Apollo or at least an extension of Apollo, so she starts talking to the raven about Troy and stuff like that. But she never finds out if it is a normal raven or if it has something to do with Apollo. In fact, she occasionally wonders if it's the same raven. It could be another one! Imagining her with Cestrinus on her lap while showing him to the raven like "this little one is Cestrinus, my son with Helenus. Although, if you really are Lord Apollo, I think you already know that. Anyway, Helenus is very happy with Cestrinus, Deidamia told me that she doesn't think she'll gave Helenus a son and Helenus kind of thought he'd die without being a father and was trying to make peace with that. Helenus told me he would introduce him to you, that is, if you really are Lord Apollo…now that I think about it, that kind of makes it unfair for me to be showing him to you, my Lord, doesn't it?" (about Cestrinus, check Pausanias).
And no, I didn't think about whether the raven actually has anything to do with Apollo. That will be a mystery until the end of time. It could have been Apollo, it could have been a random raven. Who knows? Andromache will still call it Lord just in case.