Inference is a logical conclusion based on the information provided, while generalization takes that conclusion and applies it to other similar situations. Based on those definitions, we can determine if each of the statements is a rasonable generalization or not.
"The sibling rivalry is due to the arrival of a newborn baby in the house" is neither an inference nor a generalization. There is no indication in the text of a new baby.
"The speaker is from a large family" cannot be inferred either, as the narrator only mentions one sibling.
"The speaker loves the brother" is a fair inference based on the text. The narrator mentions that her brother means the world to her, so this statement is a logical conclusion.
"The brother gets into trouble often" is not a reasonable inference nor generalizatino. The only information provided is that he insists on reading his sister's diary.
"The speaker believes others feel the same way as the speaker about their diaries" is the only reasonable generalization. The narrator assumes that by telling her brother her diary is boring, he won't want to read it. That he is only interested in knowing her secrets. Therefore, she thinks others view diaries the same way she does.