18-year-old proclaims that he'll cut off his 5-year-old half-brother if he grows up to be like his dad: ' already think of him more as my half-brother than a sibling anyway.'

Is he being dramatic, or does he have a point?

It's not worth trying to force a familial dynamic that doesn't happen naturally. It doesn't work when parents try to make their new partner a surrogate parent without any consideration for the amount of time their kid has known their new partner. No child is going to treat somebody they've known for a couple of months or years the same way they treat their parents, whom they've known for their entire life. It's just totally unrealistic.

Adults wouldn't be able to treat random people they barely know as if they were their kids, so why would they expect it from their kids' stepparents? If your 16-year-old son brought his new friend Kyle to your house and told the whole family, "Kyle's my brother now. If you don't treat him like your son and brother, you inherently disrespect him and me." Meanwhile, you don't know this kid, don't particularly care about him, and don't accept him as part of your closest kin, yet there he remains. This situation doesn't happen with kids because they have less power within the familial unit. There is no shortage of parents who bring a new person into their family without consulting their children and immediately expecting them to treat this person as a family member. It's ludicrous! 

You wouldn't expect teenage stepsiblings to act like siblings, but you might expect it from half-siblings who share the same mother and live in the same household together. The older brother in this story barely sees his younger half-brother as family, and he's planning on cutting him off as an adult if he starts to resemble his stepfather too closely.