I (22f) said that I didn’t mind talking about sex with my BIL (33m), but now I regret it. How should I stop the discussion of this topic?

I (22f) have a brother-in-law (33m). For context we live in different cities and only meet in person like two times a year. One time he asked me to look after my nephew, because he and my sister wanted to have some alone time. He was kind of embarrassed about telling me that so I told him that it’s OK. I mean, I’m an adult, they have a son. I know how it’s done. I told him (which I now regret) that I didn’t mind talking about sex with him. I did consider him a friend and I talk with my friends about sex from time to time, so I didn’t really see a problem with that. He also told me, that I should tell if something felt uncomfortable for me. But then he started messaging me in the spring, while my sister and nephew were visiting my parents in another country. And he started asking me questions about my sex life: how many partners did I have, do I enjoy threesomes, do I enjoy anal, did I masturbate today and so on. He worded them in such gross ways, that I honestly started getting the feeling that he was getting off in that. And he tried to push me to answer, even when I said that I wasn’t comfortable answering something. In the end I just lied to avoid more questions. Sometimes he also wanted me guess something about him: how many women he slept with, what woman would he like to have a threesome with. I told him many times, that I didn’t want to guess, that it wasn’t something I was interested to know. He still insisted that I guess In the end, I told him that the questions are making me feel uncomfortable, he doesn’t know when to stop questioning and that we should refrain from the topic of sex. It seemed like he understood, he stopped asking me questions and deleted all our messages from that period. But my sister is visiting our parents again and yesterday he texted me again. It started innocently enough, he was asking if I could look after my nephew when I was visiting. But then the questions started again. The last straw was when I told him that I was going to sleep and he asked whether I would masturbate before that. I told him that I was too tired and just wanted to sleep. To which he replied, that it’s a good thing to relax and masturbate tomorrow morning. That grossed me the fuck out and I reminded him that we agreed to stop talking about it. He said they he “must’ve forgotten”. Is he really getting off on that or am I just imagining things because of the terms he uses? We talk about sex differently, and while I prefer to use more medical terms, the way he words everything just grosses me out.
TL;DR: I said I didn’t mind talking about sex with my BIL, which led to him asking me invasive questions about my sex life. I felt uncomfortable and eventually told him to stop discussing sex, but he continued to bring it up again during subsequent conversations.