Introduce your stepsibling as your brother if that’s how it feels. Words can catch up to what’s already true.

Families get built in the ordinary language people use about each other. Introducing somebody as your dad’s wife’s son is accurate and also holds him at arm’s length every single time you say it, while the shorter word puts him where he already is in your life. Nobody is being lied to; the words are catching up.

Use whichever word matches how it actually feels, and let it be inconsistent for a while. If it feels wrong, do not force it; if it feels right, stop explaining the technicalities to strangers. What you call people slowly becomes what they are to you.