I know how you feel but instead its with my mother-in-law. She shows up unannounced, calls all the time that she wants me to bring the baby out and makes comments about me spending time with my sister, and her watching him. I am not working right now so there is no need for her to babysit, and I told her my sister is the only female family member I have here. We too like to spend our weekends alone as a family and her and my father in law both just show up at my house. My LO's father works long days so he has told them several times that we want space so he can hang out with our son. They could at least call before showing up.
If I dont answer the door she will call when she gets home or will sit in my driveway and call thinking I will answer my phone. I have never been a big fan of her, she is very pushy and thinks that she does no wrong. I am sorry that you are going through that with your sister in law I hope she will get the hint. Unfortunately my mother in law has not yet. Just dont stress over it and be firm with her. Good luck.