All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you had agreed with your sister’s husband on a salary or that it is known from the customs that the one who does your job is paid a salary, then you have the right to ask him to give you what you deserve for the whole period you worked for him, and if he refuses and you are unable to get your right in a legal and permissible way, then it is permissible for you to take it from him without his knowledge provided you only take your right without any excess, and your right is the salary [the fees] of the people who do the same job like you.
However, if you volunteered to work with him, then you do not deserve a salary from him for the previous period, but in the future [from now on], you may agree with him on a particular salary or that you stop working for him.
Allaah Knows best.