This title should be STEP SISTER. Not SISTER IN LAW
Here's the definition of sister-in-law in English dictionary
https://dictionary.cambridge.org/dictionary/english/sister-in-law
the wife of your brother or sister, or the sister of your husband or wife, or the wife of the brother or sister of your husband or wife
Step sister definition is here
https://dictionary.cambridge.org/dictionary/english/stepsister
not your parents' daughter, but the daughter of a person one of your parents has married
The pages said that MC's father remarry, and those two girls will be his sister, also MC has no siblings. That means, they are step sister.
Sister in law is a call for the wife of MC's brother (if any) or sister (if your sister is lesbian)
Whoever writes the english title, english description and english translation, should change the title