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. 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!