If I buy a iPhone from my brother unlocked and I have a moto g do I have to get a new plan and a new sim card ?
No à New plan is not needed. The Sim cards might not fit. You can use gsmarena to see which one you have takes. If its a newer one it's likely a nano and you'll need to cut your current card with a stamp (and be gentle as nano spec is thinner but the tolerances are generally ok) or if you're even if little uncomfortable just get a new one. Or if you have on of those 3-in-1 perforated ones just snap off whatever to get to nano unless you have a latest Moto g.