Completely anecdotal but I was able to add my brother-in-law to my Steam family without any problem and he lives about 125km from me.
The requirement is absolutely something more arcane than "same household" and Valve are keeping quiet on the actual specifics. It's possible that the fact that I've been there multiple times and have logged into Steam on their wifi in the past was enough to confirm that this is a place with close relation to me. Who knows though.
This sounds more like the infrastructure in your area just isn't up to delivering those speeds, regardless what the last mile to the home is.
I promise you Steam's CDN absolutely can deliver more than 450Mbps. It regularly maxes out my 1.5 Gbps at home, and I have no doubt that it could potentially go even faster than that if I had a better connection.
Like plugging a 10Gbps network switch into a 100Mbps gateway, it sounds like a fast final link to the home is being choked out by poor infrastructure in the region and can't be fully utilized.