A-Lilacsong

 I haven't drawn any of them with cutie marks because while I have made a design for what Amber’s would be, I haven't figured out one for Sofia or James. Because I think their futures are ambiguous enough that it's hard to lock them into one or two specific talents or character traits that a cutie mark could represent. Looking at the show itself and imagine that everything is just the same except they all happen to be ponies, I think that amber would earn her cutie mark in "Clover Time" where she discovered her love of astronomy and the cutie mark would also represent her determination and I for details which would also benefit hurt as a ruler.

 Meanwhile if we're going with the idea that James has some kind of knight/protecting others talent and cutie mark Then I think there are two episodes where it would make sense for him to possibly earn his cutie mark, either "The Silent Knight" or "A Royal Wedding" where he shows his interest in being a knight but also learns lessons that are relevant to it. But I also like to think that with James there's a possibility that he might not become a knight and lean more into his interest in acrobatics / entertainment he's shown in a few episodes would also be interesting. That or use those interests to become a unique type of knight, one that's more spontaneous and sillier like his aunt as an adventurer.

 With Sofia I don't think there's any one point in the series where it makes sense for her to earn a cutie mark. She has big moments of character development and helping others, but I think that she is still learning about the world and herself and what she wants to do with her life and I think there's a lot of room for her to grow. Even by the end of the original series, she wouldn't have a cutie mark in my opinion so maybe in royal magic there be a more logical point for her to earn one.

 In thinking about the kids specifically as ponies I don't think they would often go out of their way to try to earn their cutie marks before they have them. Like maybe one or two plot lines where they do something specific but nothing as obsessive as the cutie mark crusaders were about it. I don't see any of the other kids bullying them about it so they wouldn't have as much motivation to try. I do think though that Amber would earn her first no matter what and then she would feel quite proud about that. Then she would drag her siblings into specific situations to try and see if they learn theirs because she would appoint himself a cutie mark expert now that she has one before them.

 Also I decided for this AU since it's not just My Little Pony or Sofia the First, it's a blend of the two, that I would make up my own rules for alicorns. Roland is an alicorn specifically because he was crowned king, the coronation had magic that gave him a horn. Before then he only had wings. Aunt Tilly only has a horn because she never became queen and she is fine with that. Any crowned ruler of Enchancia will be made an alicorn and most other ever-pony kingdoms also have this as a tradition. With Amber and James when one of them eventually gets crowned they will either get a pair of wings or a horn respectively but only if they're officially crowned (ruler for a day doesn't count). So that is a little bit of extra incentive for becoming the ruler but not as much as the power the position brings.