What do you think happened to Stella, since she hasn't appeared in the trailers for a while
This is why people think Luna is Stella. Because Luna has showed up, Stella hasn't, and they look similar. Similar enough to be well within the realms of 'character redesign'. Now, I don't think they're the same. Seems weird to get people invested in a character just to arbitrarily change their name. I concur that they are probably sisters.