Is she a neighbour of yours or just friends with your boyfriend’s sister? If she’s a neighbour, it wouldn’t be too unusual for her to sit on the porch.
It’s not her you have to worry about trusting. You’re not in a relationship with her. Do you trust your boyfriend? This is the question you should be asking yourself.
If it’s bothering you that much, have a chat with him about it. He might not appreciate being told that you don’t want him talking to her, so just be prepared for this. If a man told his girlfriend that she wasn’t allowed to talk to another man, people would be calling him controlling.
I wouldn’t recommend speaking to her directly. You don’t really know her and you’re new to the neighborhood. You don’t want to make any possible problems for yourself Your sister in law is friends with her, so she probably felt she could confront her about it.
I guess all you can do is tell him your concerns and see how he reacts.