You shouldn't say mean things to family.
Lucas was vising his family, and he saw they were treating his siblings much better than him.
Lucas wants to point this out to his family.
Lucas explains they do things for his siblings that they don't do for him.
Lucas speaks loudly to his family about how much better his siblings are treated than him.
Faced with logic Lucas's family apologizes and tries to explain themselves.
They loudly deny it and get into an argument with Lucas.