Possibly. If you are not homeschooling any children of your own and your sister designates you as the person that she wants to homeschool her children then the answer is “yes.”
However, the statute reads: 115.001 (3g) “‘Home-based private educational program’ means a program of educational instruction provided to a child by the child’s parent or guardian or be a person designated by the parent or guardian. An instructional program provided to more than one family unit does not constitute a home-based private educational program”
So, if you are homeschooling children in your household then you cannot homeschool another family’s children at the same time. Doing so would be providing an instructional program to more than one family unit. The two of you could look into starting a private school.