Ultimately I think it's something I'll have to internalize and learn to live with since I'm helpless and powerless to make him change.
This. You can't make him care, no matter how much you want to.
I scan my local craigslist to keep an eye out for cats in desperate need and I see the same disappointing stories over and over again. People dumping cats because they have a new puppy or baby. Because they move and can't be bothered to look for a place that accepts pets, or won't pay a deposit. People dumping cats because the cats are pregnant. People dumping kittens because they don't want them but won't spay their females or keep them indoors. People dumping cats because they're "too old." Because they don't get along with the new kitten. On and on....
I also volunteer/foster for the local shelter. I see a lot of kitties in sorry shape and/or who were betrayed by their people.
It's really hard for me to fathom and sometimes the despair is overwhelming. I will never be able to help them all. I probably won't even make a dent. All the neglect and cruelty in the world...how can I stand up to that?
By doing the absolute best that I can for the ones that I can. Whatever happened with that cat your brother in law abused happened long ago. Whatever the outcome was, you can't change it now. What you can do is your best to help the ones you can.
Take excellent care of your kitties. Make sure they never want for anything, never give someone the chance to abuse them, and make sure they are well loved and know it.
Lead by example. You've already educated your family. You've started the conversation. Maybe in future they will come to you with a cat or pet question. You've already changed the way they think about cats. Giving good advice, and being credible enough that people take that advice, is a way to help even more cats. I think that's why a lot of people are on this board.
Have a little bit of faith. I foster, and I can't keep every cat I foster, much as I'd like to. I do my best to screen and adopt them into great, forever homes. There's only so much I can do though. At some point, you have to have faith that there are good homes and good people out there who love cats too, and will take good care of them. Talking with other cat people is reassuring. It's nice to know you're not the only one fighting this battle.
Good luck with your family! And go love on your kitties!