I’ve used IPSec to connect to my home (and office when I had my own company) networks. It has never failed me and the client is built right into my iphone and mac.

My brother and I have had several businesses together since 1994. One of our attorneys commented recently that we are so unusual because every other family business he’s represented where siblings are the owners have ended badly in terms of the family relationship.

If you’re feeling like it is not an even workload or that he’s all in like you are, then please listen to your instinct and put an end to it now. Your relationship to family is much more valuable.

FreeNAS can do it (either via AFS or SMB). Or you can just set up a Samba server with the right magic incantations and use that network share to do Time Machine. I use FreeNAS since I already have it on my network. I gave each Mac its own ZFS volume so I can limit the size independently.