Sibling love

I really hope these two have a good relationship as they get older, right now they are the best of friends. Tsuki loves kissing and cuddlings with Kaison, Kaisons face lights up when Tsuki smiles at him and he watches her all the time. Its so nice to see. But Im so worried that they will become competitive or mean, or both.
I say this because I had/have a....difficult relationship with my brother. I wont go into why but I do. I just want them to be more like David and his siblings, maybe its because theres loads of kids on Davids side? They all have multiple siblings, I guess theres more chance of having siblings you get on with if theres more of the right?
I want them to be the sort of brother and sister that call each other just for a chat, that remember each others birthdays, that are KIND to each other. I sort of want to pause time on thier relationship right now, keep it like this, just like this.