Hopefully I can help with some perspective, having familiarized myself with my in-laws beliefs.
“for all the land which you see I will give to you and to your descendants for ever. I will make your descendants as the dust of the earth; so that if one can count the dust of the earth, your descendants also can be counted.” Gen 13:15-16
The big sticking point, from the likely premillennial perspective, for your brother-in-law is the “for ever.” That it is tied to “land which you see” further complicates the issue, because instead of taking these things literally, they are being taken literalistically. Literally, there is one place that is forever and that is heaven. So literally, descendants that could live in any land forever would have to live in the only literal and eternal place, which would be heaven.
Basically, because your brother-in-law is likely viewing this through a premillennial lens, he is seeing this as unfulfilled. It also helps to know that he considers Israel to be Israel, and the Church to be the Church and ne’er the twain shall meet. This also complicates the issue.
So, Israel was given the promised land, over which they had dominion at it’s height under David and Solomon. Then the tribes fell and the whole thing went kerplooyee, as it always goes on earth. So it appears to be unfulfilled because they had it all, except that it wasn’t eternal, so it couldn’t be fulfilled.
The clincher is the seed mentioned (from the beginning to Eve) is Jesus (Gen 3:15) and the eternal kingdom which is promised is the Church (Matthew 13, which sets the kingdom up as: visible aka: united under **one **banner, with hidden treasures aka: **holy **sacraments, vast/large in an invisible way aka: universal, not “national”, visibly flawed from an earthly perspective aka: apostolic, let by man) and this kingdom is not Jews vs Gentiles (Rom 11:19-26), but has descendants as plenty as “the dust of the earth” (Rom 3:22-23.)
So the kingdom is fulfilled on earth because: “And so I say to you, you are Peter, and upon this rock I will build my church, and the gates of the netherworld shall not prevail against it.”
And Israel, which is also Jesus (Ex 4:22; Hos 11:1), is also the Church (Acts 9:4.)
Hopefully that helps.