BUT: It's a bit strange that you can just randomly affiliate to a family and than have these features with them but not with the family of your spouse. IMO the entire thing should simply be merged with marriage (and maybe an option to turn them of for people who dislike the affiliate features but still want to marry).
Then you would also really belong to a family and not just be obscurely "affiliated".
EDIT: Now I've got an even better idea
What is annoying me is that you do not show up as part of the family in character relations with your affiliated family and don't get the features with the family of the spouse. One fix would be merging the whole thing with marriage as I said but that would be limiting.
Another option would be adoption. As the game does not seem to have "grandfather" relation strings you should only be able to be adopted by the "father" of the family.
This is also realistic as younger lords who don't have their own children yet would be unwilling to adopt someone and only the head of the family should have the authority to accept someone into the family.
The relation with your new father should be the most important but positive/negative relations with your new brothers will give bonuses/mali and could push the acceptation/rejection over the edge.
You will than show up in the relations as son/daughter and will have brothers/sisters
If you then marry someone you will all become one big family fixing the problem of the lacking affiliation features for the family of your spouse.