Kid lashes out because Onee-san is talking about something the kid feels he has no control over seemingly due to family problems - both parties went overboard. Onee-san should've had more tact, though we can't fault her because she lacks the information we do i.e. family issues the kid has, and the kid shouldn't have lashed out to someone who meant no harm and is speaking from their heart. Keep in mind that these two people are still basically strangers to each other and neither knows much of anything about the other - shitting on either one of them is retarded and shows you lack empathy and aren't coming from the perspective of a mature adult who has lots of experience interacting and/or raising kids. Criticism is the last thing this kid needs and Onee-san seems to be aware of that which is why she gave him space after his uncharacteristic outburst.