Dealing with changing family dynamics is never simple, especially when blended families are involved. No two circumstances are going to be the same owing to the complex natures of the individuals involved. No two relationships are the same, so you can’t really expect that someone will treat two people exactly the same, either. This unrealistic expectation is one that parents struggle with in parenting two different children “equally.”
Emotional closeness isn’t something that can be forced either. While it’s true that spending more time with his stepsister might foster a stronger bond between them, it could also cause things to go the other direction. But it really sounds like the dad is just mad that the teen has elected to spend more time at his mother’s house. Instead of being mature about handling his feelings, he is finding some way to attach that feeling to something he knows he can use to guilt-trip the teen into not spending as much time there. So, the reality is that this whole situation is more about the dad’s control over the teen than it is about sibling dynamics.