Hi, Ilaria! I really like where you are going with this portrait. The poses are winsome yet not "too cute" and I love that they are sitting on the floor. I think you have done a good job with having neither of them steal the show from the other, which sometimes happens in a multiple portrait. I think you have good balance here and the physical closeness connotes a comradely feel to their relationship.
You have a wonderfully fresh painterly approach.
I know that you plan out your compositions carefully so I'm betting that you have planned on having all those lines converge on the little sister's face. The only line that is bothering me is the dark vertical line of the sofa arm which could maybe be blurred and lightened.
I have a question about the scale of the sofa in the background: is it one of those child-sized sofas? I'm also wondering if you want to bring up the color just a little bit on the background to make it less gray. (Speaking from the angle of an artist in fuschia Phoenix to an artist in foggy London.
)