There are lots of people out there who find comfort in believing that an 8-week old fetus (or a 10, or a 14-week old fetus) is "just a ball of cells" when they make a decision for a termination. After studying embryology, and especially now that I am doing midwifery, I was actually shocked to learn that an 8-week-old fetus looks just like a tiny baby and has almost every structure and organ pretty much formed, they just need to grow and differentiate for the rest of the pregnancy. After supporting a woman through an unwanted termination at 18 weeks (early rupture of membranes due to a uterine infection) and catching that tiny baby with my own hands, and dress her, and take photos of her for her parents, now I can never believe or say that a fetus is "a ball of cells" anymore. Considering that I never wanted children, had I fallen accidentally pregnant when I was younger I would have had a termination, blissfully unaware of how the fetus would look like, or how developed it would be. And by now I would have been horrified to learn what I had done.
So in this particular situation, OP, I can understand both sides perfectly. I understand why it feels unfair to you that you are asked not to discuss your pregnancy in front of your DB and SIL, and I also understand that they were the ones who decided to terminate the pregnancy. But I can also understand why they would not want to learn the truth about 8-week-old fetuses, and how they have fingers, and toes, and their heart has been beating since day 22-24, and they move, and how finger and toenails appear at week 12. I know they had a termination and that is the truth... but I also understand knowing the truth is not always the best thing.
As others have told you, there is nothing stopping you from discussing your pregnancy with your parents. Avoiding these sensitive issues around your DB is not unreasonable, and it does not take anything from your enjoyment of your pregnancy. Surely you would not be wanting to share these details knowing that the person listening is feeling like crap, especially it being your brother?
Or is there a scent of "they killed their baby, they very well deserve to know about this" in this story? In which case, YABU a thousand times.
What if - God forgives it - you lose this baby in these early stages? How would you feel when a pregnant friend came to you all excited to tell you about the developmental stages of her baby?