Remote access to iMac from Ipad

This is absolutely doable, and you’re on the right track. The easiest way for your brother to access his iMac from his iPad is by using Apple’s built-in Screen Sharing together with a simple remote-desktop app. On the iMac, go to System Settings > General > Sharing and turn on Screen Sharing. Then, on the iPad, install a VNC app such as VNC Viewer and connect using the iMac’s Apple Account (aka, Apple ID) or local network address.

Once connected, you’ll be able to see and control the iMac right from the iPad. For screenshots and creating “white pages,” the iPad makes this very easy — take a screenshot, then tap Markup to write or draw on it, or open the image in Notes or Pages to create clean, white documents with text and annotations.

you can try to look at the teamviewer app, it should let you remote desktop from an ipad to a computer.