I think that even though his sister, could be concerned, most part of what Ichikawa was saying, is projection. He still insecure about himself and have fear of what other people would think about his feeling for Yamada.
that reminds me. i was hanging out with my current gf before we started dating. Happened to see a friend of mine out who asked if she was my gf. I said “not yet” without noticing lol