If you just mention the siblings at the beginning along with everyone else, it's not really much different than having them show up except that now all the readers have is one more name they don't know whether to file away or not. The best plan is to have them mentioned when it's natural. If they're important enough to show up at all, you don't want one just suddenly being there halfway through the book with no mention at all, of course.
Think of introducing your characters like how you're introduced to people. Let's say you're at a party and your best friend introduces you to Joe. Now you know Joe, and maybe a couple of things about him that your friend or Joe told you during the introduction. "Here's Joe. He's an accountant. He went to college with me. Hey, Joe, remember when we rocked that protest to save the spotted owls?" As Joe interacts with people that you are interacting with too, you learn more about him. Maybe he tells stories about his life. Maybe he mentions some family members or friends. If he's telling you everything about himself, whoa, overload, you're never going to remember most of that stuff, especially names, and especially on top of the other people you're meeting at that party. And he's probably not going to talk about every single person in his life.
But you and Joe hit it off, and he starts becoming more important to your life. You start learning more about him and his history. He invites you to eat dinner with his family. When you walk in, chances are you're not going to be surprised to see that he has two brothers. Chances are that he's mentioned them in context at some point. It might have been at that party where you first met, but if he talked about everyone he knew right then, you're not going to remember. But if one day between the party and Thanksgiving dinner, you're meeting him for coffee, and he's sighing because his brother is calling with tax questions again, then you'll probably remember that first time you went, "Oh, he has a brother." And when you meet that brother for the first time at that dinner, it won't seem strange at all. If you've never heard of the brother, it seems very strange (and probably makes you wonder what's wrong between them that he's never even mentioned the brother).
Of course, if the siblings are very important to the story, I would assume they'd be introduced ASAP and not left until late. The OP is vague on what "later on" means. If it means, 5K in instead of on the first page, I wouldn't consider that too late for introducing much of anything, actually.
