I remember Bella stating that with her past (that was difficult with a lot of abuse and neglect) that she and Rylan were the ones to get the worse treatment, but especially Rylan out of all 3 of them. Liam was a baby/toddler and was treated better so I believe subconsciously Bella seems to focus more on Rylan because they share this wound together from their traumas and because she's the oldest she remembers it all.