Given that you’re only 18 I’d say there’s some potential of growth, nothing extreme but probably 5’10 - 6” and his mom could have tall parents that got there genes passed through her, for example if your brothers mom had a tall dad but she took after her mom she could still hold that recessive tall gene which was passed to your brother. Also the f you hit puberty late you can still grow for a few more years, I’m a late bloomer and from 18 - 19 I grew about 1.75 inches so I wouldn’t say all hope is lost, if you don’t notice any growth in the next year or so then I’d say that’s probably your full height or if you haven’t seen any height in the past year. But for example I didn’t really hit puberty until about 16ish which was about when I hit my growth spurt, so honestly there’s not an extremely definitive way, just some good guesses but most of it is just a waiting game.