I use the MoCA 2.5 adapter by goCoax. I won't say I recommend it since I haven't compared it to other adapters, but it appears to do what I want it to do and I get about 2Gbps through the adapters. Every one I could find seemed to be around the same price point ($60-$80 per adapter) so I just went with a well reviewed one.

I would definitely recommend if fishing cat6 is not an option and you would need to open up walls. Unless your BIL has a lot of kids and their friends over who all like to stream different 4k movies from a NAS, I can't imagine any normal user with 1gbps WAN finding 2gbps LAN insufficient.