Where the hell were they? You would think that they would show up when Ariel was showing everyone her baby, or when Ariel came back to the sea, or right at the end when the wall between the castle and the sea was gone! I mean, what the f!
And I will get him back even as he gloats. In the meantime I'll practice on less honorable throats!
They are there. In the beginning, when Ariel shows her kid the sisters are singing the line "Up to the sea". However, it is just a cameo, but you see them if you pay close attention.
You also see them at the very end when all merpeople and human beings sing "Here on the land and sea" but they always are in the background.
It would have been great if Ariel and her sisters had searched for Melody together.
If Ariel is around 28 in this movie, her sisters are 29, 30, 31, 32, 33, and 34. You probably don't see them because they've moved on and hitched up with Mermen and had kids of their own. The oldest is heir in the throne though, I suppose it would make sense for her to get involved when Triton does a massive search for Melody.