This unique type of partner training can be dangerous if not done correctly. We're building muscle and garnering strength, but we're also starting with a very heavy load on the bar. The person lifting has to be engaged and completely focused at the start, but so does the spotter.
As the spotter, it's important to make sure you're completely there for your brother in iron. Keep your hands on the bar, and provide support for the duration of the set. Not sure how much assistance to give? Pay attention to how much effort the person pressing is using and adjust as needed.