1
someone's stepsister
stepsister 是可数名词，前面常用所有格限定词。
My stepsister lives in Shanghai.
我的同父异母（或同母异父）姐妹住在上海。
异父或异母姐妹
stepsisters — plural of stepsister
先理解核心含义，再看它在真实句子里的用法。
异父或异母姐妹
a sister who shares only one biological parent with you
My stepsister lives in Shanghai.
我的同父异母（或同母异父）姐妹住在上海。
继父或继母与前夫或前妻所生的女儿
The daughter of one's stepparent by a previous relationship.
父母的继女,且非自己的同父异母或同母异父姐妹
The stepdaughter of one's parent which is not one's half-sister.
My stepsister and I have the same father, but we grew up in different cities.
我和我同父异母的姐妹有同一个父亲，但我们在不同的城市长大。
She invited her stepsister to the family dinner last Sunday.
她上星期天邀请了她同母异父的姐妹来参加家庭聚餐。
Many children learn to share a room with their stepsisters.
许多孩子学会和他们的异父母姐妹共用一间房间。
stepsister 是可数名词，前面常用所有格限定词。
My stepsister lives in Shanghai.
我的同父异母（或同母异父）姐妹住在上海。
若需说明是哪种情况，可在上下文中说明是同父异母还是同母异父。
She has a stepsister from her father's second marriage.
她有一个同父异母的姐妹，是她父亲第二次婚姻所生。
复数形式 stepsisters 表示姐妹双方，常与 be 连用。
They are stepsisters because they share the same mother.
她们是同母异父的姐妹，因为她们有同一个母亲。
Not this
My stepsister and I have the same mother but different fathers.
Use this
My half-sister and I have the same mother but different fathers.
stepsister 是继亲关系，指父母再婚后成为的姐妹，与说话人没有血缘关系；half-sister 才表示同母异父或同父异母的血缘关系。因此表达血缘关系时要用 half-sister。
Not this
She is a my stepsister.
Use this
She is my stepsister.
英语中 a 和 my 都属于限定词，不能同时修饰同一个名词。表达“我的继姐妹”应直接说 my stepsister；如果想说“我的一个继姐妹”，要用 a stepsister of mine，而不是 a my stepsister。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
我的异父或异母姐妹
My stepsister is coming to visit us this weekend.
我的异父或异母姐妹这个周末要来看我们。
异父或异母姐姐
Her older stepsister works at a hospital.
她的异父或异母姐姐在一家医院工作。
异父或异母妹妹
My younger stepsister is very good at math.
我的异父或异母妹妹非常擅长数学。
Use stepsisters naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
把 step 想成'台阶'：你们只共同踏上一个台阶（一位父母），所以是异父/异母姐妹。
语域：neutral 常见场景：家庭关系讨论、日常对话 stepsister 是中性常用词，通常用于说明家庭关系，正式或非正式场合都比较常见。
stepbrother、stepmother、stepfather、stepparent 和 stepsister 一样以 /step/ 开头（重读音节相同），但它们并不与 stepsister 押韵；sister 和 half sister 共享 -sister 拼写，但重音在 sis 上，读音明显不同，注意区分。
Dictionary facts from ECDICT, WIKTIONARY.