Find the shared activity
Many hikikomori aren't fully shut in — they may play games in the living room or come out at night. Find one thing you both enjoy (a game, a show, a snack ritual) and just… do it together. No agenda.
Small Ideas
You don't have to fix anything — you're the sibling, not the therapist. These are just ways to share space and time.
Talk about anything else
You might be the only person who talks to them about normal stuff — a meme, a song, gossip about a game. Being the one person who doesn't bring up The Situation is a genuinely important job.
Leave things, not lectures
A manga you finished, a snack you bought two of, a sticky note with one dumb joke. Small deliveries say "I think about you" without cornering anyone into a conversation.
Live your life, guilt-free
Going out with friends while your sibling stays home is not a betrayal. You staying small doesn't make them bigger. A sibling who is okay is one of the most hopeful things they can see.