What do you mean "if"? There's no way they're not having sex (provided this game lives long enough to get to that point) and I don't see it being sad at all either. Just like with Ellie, Sophia will start to want it more and more, particularly if she's not having sex with Liam, and we've already seen instances where Sophia, despite finding it somewhat awkward, has actually relished and even enjoyed the attention and affection she gets from Dylan. When they eventually have sex (not if), it will probably be a very touching and emotional thing because we know how much Sophia loves her kids, and when that love becomes sexual and she views it as just means of showing that love and strengthening that bond, it will be both enjoyable for her and a really passionate moment for both of them, same goes for when she has sex with Ellie.
Also, I don't see Dylan as being the final step on Sophia's journey as there's still scope and opportunity to turn this into a whole family (minus Liam) affair, with Sophia, Dylan, Ellie, and possibly Patricia all getting in on it together. Sophia could have sex with Dylan, Ellie, and Patricia separately, but there's still the potential to expand that to Dylan/Ellie, Dylan/Patricia, Ellie/Patricia, Sophia/Dylan/Ellie, Sophia/Dylan/Patricia, Sophia/Ellie/Patricia, Sophia/Dylan/Ellie/Patricia. Maybe there could also be Sophia/Dylan/Sam for people following that path, as well as Sophia/Ellie/Julia, Sophia/Ellie/Julia/Patricia, Sophia/Ellie/Sam, Sophia/Dylan/Emma, Sophia/Alyssa/Patricia etc. There are lots of opportunities and possible pairings/groupings that it shouldn't end after Sophia's had solo sex with the principle cast.
This is where I feel the game should be by the time we get into the 20s of playable days. Maybe by day 23 or 24 she's had sex with all the principle players (Dylan, Ellie, Patricia, Sam, Alyssa), and the game can then start to open up to some other characters joining in with Sohia and these characters.
If there's any "sunset" for Sophia, it's either going to be with Dylan, Ellie, Alyssa, Sam, or Liam, or a combination of some of these characters. I don't see an ending for her and Patricia because Patricia's married and living a good life which she's not going to give up to shack up with her sister for the rest of her life, and I don't see Sophia and Carl having some kind of sharing arrangement with Patricia either. So, other than these 4 characters, there really doesn't seem, to me, to be anyone else that Sophia could have an endgame realtionship with, if that's how this game will go.
The other option is that Sophia will do all these things with all these different people, Liam will be the none the wiser, but then Sophia and all of them will go their separate ways after she's had this wild time of hedonism and debauchery, and stay with Liam until her dying day.