Mhhh 'bro' doesnt mean that she cant love him. Actually I think its a good and legitimate way for Cyanne to become more intimate with Leto without getting too obvious. So it kinda fits to her shy role actually imo as bro would be the closest thing she would be able to say out loud. Also her feelings are quite unsorted I suggest so she doesnt really think that active about her relationship with him. She notices that she has more feelings towrads leto in situations but then throws the thought away.Hmmm, I don't know about you, but I have a hard time portraying Cyanna calling Leto "bro" or brother".
Not only would she feel a bit too shy about it, but it further reinforces the idea that they are brother and sister, and that her love for him is wrong, making her feel bad about it. If it makes her feel bad, then she isn't likely to say it if she doesn't have to. Same for Haylen, she doesn't strike me as someone who would directly refer to Leto as "son", but more as "honey", "sweetie", etc... "Son" sounds more like what a dad would say, but maybe that's just me?
For Rose, I could add more "darling" and the likes, it would fit her personality pretty well.
Mh no I actually think it would help us to give us more of a mom feeling with her, because after all she only adopted him and always felt very distant from him, even though she was always weet towards him). But maybe son only fits in certain situations, e.g. this sofi is my son leto. Or I am so proud that my son is a famour guard now. Things like that. Other then that you could go with, 'my sweet, little boy' or just in general give more remarks about Haylen really accepting Leto as her son now, cause it is not natural