Boyfriend wants to hang 1o1 with another girl

My boyfriend, let's say A, wants to go hang out with his friend N downtown and at his house. They are really close and he says "she's like a sister to me." A few years ago they went out for 8 months and ever since have been really close. N is extremely horny & constantly posts about sex, and cheated on her boyfriend. I told A that I'm uncomfortable with them hanging out alone. He understood but said i was overthinking.

Then, he said he'll hang out in a group with her. He never brought up who was there. He also said, "she's leaving soon, so can N and I walk to your house?" This made me feel as if they hung out alone behind my back.

Am i being lied to? Am i wrong for preventing it?