Well I think Noctis and Stella were childhood friends so if Luna is Stella's sister it would make sense that they all knew each other growing up as well. We've seen that Stella fights Noctis in the early trailers, and in this new one Luna managed to escape somewhere and seems to be helping them. That's about all we know really which isn't much to go on at all. I don't think there will be a triangle though, not since I have a feeling Noctis and Stella will be more like star crossed lovers since it feels like she's going to fight Noctis. That really would be overkill.