Oberon tells Cersci in season 4 that he has 8 kids. We only see 3 of them in the show, and while the younger kids by Ellaria are too young to really players, he does have other older daughters. Sarella Sand has never been mentioned in the show and only commented about in the books but she would be the most likely candidate of the family to be in a position to do anything. In the books she's not in Dorne, but she may be running around the Citadel disguised as a Maester in training.

I don't know if the show is able to really cover what is happening in Dorne effectively right now. It's been said before but the Dorne plot in the books has so much more meat, but with the press for time some things just have to be cut or condensed. There are other great houses in Dorne. If the Martells are truly gone then I would guess the House Dayne would be in line, but again I don't think the show as of right now has time to cover that.

Let's just say there's a big ole power vacuum and folks are trying to figure shit out, and that's going to take time.