There are tons of narrative tools and techniques that can be used to infer that something is going on with Dylan and Ellie. One that I literally put about 30 seconds of thought into;Well, I guess we'll have to disagree on that one. I can't imagine what a story between Dylan and Ellie would look like, told solely from Sophia's perspective. Even more, I think that L&P should definitely not do without this story, because it, at the latest as soon as Sophia gets to know about it, allows for incredibly interesting story twists.
Sophia notices at the dinner table that Dylan and Ellie aren't bickering as much. Option: Sophia thinks to herself "That's nice" and nothing further comes from it, or "I should ask if anything is going on" Sophia asks her kids.
If Sophia asks, the kids can play dumb or be coy about it, which peaks Sophia's interest more and maybe she does a little investigative work. Scene of her standing on the balcony and overhears a conversation between Dylan and Ellie and learns why they're getting along better.
It plays into her being an caring and available mother to her kids, it empowers her making her a more active protagonist, and it conveys what their story is without spending unnecessary time away from Sophia's PoV.