If you disable DHCP on Router #2, you effectively have turned that router into a network switch.
All computers will see each other, and everything will be easy-peasy.
BUT. . . if you want to be a network nerd, then you will run a LAN cable from Router #1 into the WAN port of Router #2, and allow DHCP to do it’s thing on Router #2
For the love of sanity - - - use a separate DHCP subnet for the two routers. (it looks like you are planning to do this from your sketch) (I have no earthly idea what happens if you try to use the same IP addresses on the two networks.)
You should be able to access anything on Network 1 from Network 2 by using the correct Network 1 IP address. (I don’t think using a “friendly name” will work. . .use the IP address directly). So if you want to see files. . . type 192.168.1.100 in a file explorer address bar.
I “believe” the visibility is one-way. . . . Network 2 will see Network 1, but computers on Network 2 will be shielded from Network 1. . .which is what I think you are trying to achieve.
If your brother wants his own privacy, he needs to install a Router #3, wired into Router #1. Again, anything on Router #3 will be invisible to users on Router #1 and Router #2.
You really don’t have to worry about port forwarding unless you are access from OUTSIDE of Router #1 (i.e. from a coffee shop down the road). The DHCP addressing from Router #1 should take care of all that.
Words of cautions:
This is not as easy as I make it sound.
There probably a few other options and settings you need to understand to make it all work in the end. . .but what you are trying to do should be readily doable.
First “gotcha”: My MyCloud device has a setting called “Dashboard Cloud Access”. It is off. With this being off, it will not allow access to the dashboard from a different subnet. In otherwords, the PC trying to access the dashboard of a NAS on Network #1, MUST be Network #1, not Network #2.
Hopefully, the two routers are physically close to each other, to make trouble shooting easier. When playing with this stuff, I take a laptop and jump to different wifi connections to test things. When I have to test connectivity from OUTSIDE the router, I use a phone hotspot to provide the outside-of-network access point