"My little brother is reading a difficult textbook."
私の（わたしのmy）弟（おとうとyounger brother）は 難しい（むずかしいdifficult） 教科書（きょう か しょtextbook） を 読んで（よんで） います。
Is it also sometimes common to say oto ha instead of watashi no oto ha?
It is otouto* Japanese often says only otouto/imouto, since its implied in the sentence you are talking about your family. Just remember some contexts can mean something different. Its pretty much unlikely to ask about your brother age for someone who is not your family for example, in this case, you were asking for someone else's age.