In my experience, the death of a parent amps up -everyone's- behavior patterns in a nuclear family. I don't know how to describe it in my instance except to say that people became more of themselves (overfunctioners overfunction, avoiders avoid, crazymakers make extra crazy), And when money is involved even somewhat functional families can have emotions running high.
So take the scene at the beach house and imagine that as a mere shadow of what things might be like after your father passes.
What your father is asking you to do, now, is to make nice with your brother - regardless of his behavior. To turn the other cheek. Because it will make your father comfortable. And your doing that now is -absolutely- no guarantee that you'll be able to do that when your father passes.
It seems like what your Dad is doing today, in the present, is emotionally manipulative. And while it might feel like a compliment, asking you to serve as executor is really a huge burden. Your brother is likely to be more difficult to deal with than he usually is, and you will be dealing with a major loss.
Are you willing to shelve/shove your emotions and boundaries now to satisfy your father that you'll be able to shelve/shove your emotions and boundaries in the future?
Hugs,
SL