Can we help? Lots of folks have answered this need, and done it well (in particular, I recommend Anthony Cody's "Ten Colossal Errors.") But your brother-in-law may require something a bit more pithy. Let me offer some suggestions.
1) College and Career Ready Is Not Enough
In its very first sentences, Common Core redefines the purpose of education. It declares that the one and only purpose of education is to get students ready for a job (because in CCSS-world, college is just a way to get higher-level job training). That's it. Anything else we ever thought education was for-- fostering well-rounded humans, preparing good citizens, allowing students to reach better understanding of themselves and their place in the world-- that is all thrown out. Education has one purpose- to prepare students for work.
Not only that, but CCSS also redefines what "college and career ready" means-- it means English and math. That's it. Want to be a musician or a lion-tamer or a physical therapist or a nurse or a machinist or a advertising executive? Your preparation is exactly the same as every other profession, and it's all about English and math.
"College and career ready by studying English and math," is a sad, tiny redefining of what it means to be an educated person.
2) There Is No Flexibility
"Well, teachers will just adjust. After all, the Core are a floor, not a ceiling. They may not be perfect, but I'm sure they'll get better."
But CCSS allows no flexibility. no adjustment, no room to move. There is no process for review and revision, no number to call with your suggestions. The Core will not get better. They will not change. The only improvements will be made outside the system; the only teachers who tweak the Core will be those who go rogue.
The Core are copyrighted. Nobody is allowed to change them, ever. You will need to remind your brother-in-law of this may times.
3) The Standards Are Bad
Amateur-hour bad. I can point you at long, involved explanations of exactly what is wrong with them, but the short explanation is that we know many things about how different humans at different stages of development learn how to read and write, and the Core ignores all of them. "Well, just add in all that stuff you know," your BIL says? Go back and reread #2, I say.
4) Standards Don't Do Anything
Some folks keep saying that CCSS will help us close the achievement gap. They have no evidence. Not only do we not have evidence that standards close the gap, but we also have evidence that they don't.
Lots of states, most states, have had standards in place for a while now, and this achievement gap that we keep talking about inside the state. Every single state has a pocket of urban poor and a pocket of suburban wealth. If standards helped with that gap, would we not see that within states?
In other words, if Dunlevania had no, or poor, standards, we would expect to find a huge gap between its urban poor and its wealthy. That could be compared to South Borgia with its super-duper states standards, and a much smaller gap between its lower and upper student achievers. But we don't see that. If there is a secret formula for using state standards to shrink the achievement gap, nobody has discovered it yet.
Or, we could look at international achievers and see that, hey, this nation with great test results also has high strict standards, and this country with lousy scores has none.
The international scene and the fabled fifty-state standards hodge podge should have provided ample opportunity to demonstrate real linkage between standards and educational achievement. And yet, chirping crickets. There is absolutely no reason to believe that national standards will improve a thing.
5) The Core Turns Schools Backwards
Under the Core, students exist to serve schools. A school needs a student to put out certain numbers, show certain results, perform in a manner that serves the needs of the school itself. Witness who schools have tried to enforce the taking of The Test-- not because they think Junior will have his growth stunted if he doesn't take the PARCC, but because the school needs his numbers. What a student wants or needs, what a student expects to get out of education-- none of that matters. The student has to show that the school is succeeding, and that can only happen if the student performs as the state says he must. What the student wants is simply unimportant under the Core.
POINTS NOT TO BRING UP
1) Wacky assignments
We really need to leave this one behind. Bad math problems, confusing readings, misprints-- these have been around since the dawn of time. If CCSS went away tomorrow, there would still be terrible number line problems and writing assignments that asked students to imagine things that their parents disagree with. Do not offer that up as a reason to revolt unless you want the revolution continuing in your classroom long after CCSS have been swept away.
2) CCSS Was Created By Corporations and Profiteers
Yes, that aspect burns my toast, too. But it's not a winner in arguments with civilians, unless you want to try to argue that you would reject the best education program in the world if it came from non-teachers. Diane Ravitch-- not a classroom teacher. Founders of BATS-- not classroom teachers.
The origin story of CCSS is important because it explains why the standards are such a hash, and because understanding the purpose of CCSS (making a buttload of money) helps make sense of how it unfolded. But something civilians seem to get that teachers are reluctant to admit-- all that stuff about profiteering and backroom deals and underhanded double deals could be true AND it could also be true that the standards are great. So don't argue that the Core has a bad pedigree-- argue that it is bad education. The pedigree just explains where the suck came from.
3) Federal Overreach
Again, I agree that this is a problem with the Core, but it's not necessarily a winner when arguing with your BIL.
The two political wings are united in one opinion-- federal power should be used to make people act as they should. Conservatives and liberals alike get this. Tea Partiers are perfectly happy with federal overreach that keeps The Gays from being, you know, all gay and stuff. Liberals hate activist judges unless they are reaching the correct conclusion.
"Federal overreach" is an argument add-on. It will get people more steamed about something they already hate, but it will never make them turn against something they actually like.
Good luck. You have till the Fourth of July family picnic to get your arguments in order.