Not "my sister room" — say "my sister's room". 's 就是用来表示这个房间 属于 她。
试一试: This is my ___ room. (it belongs to my sister) (sister's / sister / sisters) 答案: sister's — 要表示这个房间属于她，需要加 's。你漏掉了，记得补上：my sister's room.
以 -s 结尾的复数：只加 '
复数本身已经以 -s 结尾的话，只加一个撇号，不用再加 s。
my parents' house
the students' books
the teachers' room
Not "the students's books" — say "the students' books". 已经有 -s 了，后面只需加一个撇号就够了。
试一试: These are the ___ books. (they belong to the students) (students' / students's / student's) 答案: students' — 'Students' 已经以 -s 结尾，所以只加一个撇号，不再加 s：the students' books.
撇号放在哪里？
先找出所有者，再把撇号放在所有者后面。一个姐妹是 sister's，两个姐妹是 sisters'。
one sister: my sister's room
two sisters: my sisters' room
one friend: my friend's phone
Not "my sister's room (two sisters share it)" — say "my sisters' room (two sisters share it)". 在书面上，正是 sisters 后面的这个撇号，表明这个房间属于 两个人。
试一试: Mei: Whose room is this, Liam? Liam: It is the ___ room. Two students sleep here. 补全 Liam 说的话。 (students' / student's / students) 答案: students' — 房间属于两个学生，所以撇号放在 students 后面：the students' room。'Student's' 的意思是只有一个学生。
试一试: This is ___ bag. (it belongs to Liam) (Liam's / Liam / Liams) 答案: Liam's — 一个所有者，Liam，所以加 's：Liam's bag。（一个所有者加 's。）