Im SOBBING cs Lestat remembers Claudia's gait and the distinct way her heart sounded. How then Louis and Lestat r both talking abt wht is different and they both saw it, the lack of a spark in her eyes.
Louis is not able to let the facade that she is Regina and not Claudia Go because Claudia filled gaps in him, of his sister, of ruining families due to his nature and so on, and he felt the loss and was comforted by an illusion
BUT Lestat cannot indulge in this Regina fantasy because for him Claudia was a variable, her own person, who (however much hate was between them) he still treated her as a individual (teaching harsh lessons, or enjoying a killing spree) who he kinned deep down (both of them lost their first love, the assualt Magnus n Bruce, remember the scene where he screams to Armand during play rehersal that Claudia has his strength and will not cower)
This found family has me in tears istg