I agree with @DennisC, you should upgrade to 5.3.4. It's the most recent supported firmware for ISY994i. While there might be very few changes from 5.3.3 to 5.3.4 it's enough that it would be the first step in further troubleshooting any other issues you have.
As do your exact experience - your brother has the "Launcher" which loads the finder and allows the computer to download the UI to properly match the firmware with every launch. (Thus, "Launcher")
You downloaded the admin console for the direct version you installed. It would be just as though you downloaded admin.jnlp from the ISY994i directly.
These two routes to access the Admin Console act differently, but they are in fact the same once you access the ISY994i.
Since the Launcher was introduced in 2018 that is the most often suggested and the recommended process for accessing the ISY994i admin console. It was designed with 5.x in mind as beta builds were being released so users would always have firmware/user interface matching.
So nothing is "wrong" in your setup. You're just taking different routes to get to the same destination. There have been a lot of reports of the font/display to be different as we moved into 5.x. You can change those settings, but odds are they won't stay for long as they tend to be dropped and need to be reset.
But, be sure to go ahead and upgrade to 5.3.4 (since you're so close it should be very quick and not require any additional effort, but be sure to follow the steps as outlined on the release post.
Geddy's post in ISY Finder question was marked as the answer