Managing my own manual when being asked to

My brother in law and I are nothing alike. He is younger than I am by 10 years, and younger than even my sister in law by 7 years. Not getting into details or judgments, but when we are all Together I take charge, with my own expectation he will keep pace with my energy, not my actions. When his immaturity ( arguing, isolation, despondent, inappropriate comments) creates issues, my sister in law gravities to my wife for connection, support and comfort. This typically leaves me flying solo, losing my connection with my wife. Needless to say, this pisses me off. And then I become a bully and force my code on him, get irritated with my wife and sister in law, and feel isolated.

We are going on vacation for my sister in laws 40th next year, (COVID dependent), and I am dreading spending thousands of dollars to be his babysitter.

I am struggling to maintain a connection and be present, while stating a Boundary that doesn’t leave me angry and isolated