* Chwaer-yng-nghyfraith: This is the most literal translation, meaning "sister-in-law." It's often used to refer to the sister of your spouse, but can also be used for a stepsister.
* Chwaer llei: This means "little sister," but could be used to refer to a stepsister who is younger than you.
* Chwaer hŷn: This means "older sister," and could be used to refer to a stepsister who is older than you.
* Chwaer cam: This literally means "step-sister," and is the most direct translation. However, it's not as commonly used as the other options.
The best option for you will depend on the specific context and the age difference between you and your stepsister.