HYPHENS:¿¿¿¿¿sister in law/ sister-in-law, brothers in arms/ brothers-in-arms, twenty five/ twenty-five?????

My favourite students are those who have a permanent thirst for knowledge, an infinite curiosity, a passion for learning .......
when a student asks me about such apparently unimportant details like when to use hyphens, he deserves the best answer! and I think the one offered by  Oxford Dictonaries is a really good one!