Weird relationship with step sister

So my bf and his step sister have a weird relationship and I don't like it and just today he told me he asked her in the past if she was into him and that makes me even more uncomfortable. They talk constantly and he helps her around her house and will do whatever she ask. I now feel very uncomfortable about their relationship especially since he told me that. Is that weird to anyone else? I told him he needed to stop talking to her and he said he isn't. I don't know what to do but I feel grossed out by that.