I was in the same geometry class with one of my sisters when I was 15 and she was 14. I was in the same band class with my other sister when I was 17 and she was 14. At those ages, it was neither an advantage nor a disadvantage for me. We didn't sit together in class, and if we did, we wouldn't have been allowed to talk in class anyway, the same as any other pair of kids.
I believe the geometry class was extra stressful for my sister, because that subject came so much easier for me. It's easier to brush something like that off if a stranger is outperforming you. I was able to tutor her sometimes, but I think if you ask her honestly, she wouldn't care to repeat the experience.
We homeschool our children (ages 4, 7, and 10), and teach them together quite a bit, despite them being 3 years apart. It works very well for some subjects, like art, music, science, and history, and not quite as well for subjects like reading and math, although even there they can do mutually beneficial activities like my son reading aloud to his younger sister who doesn't read yet. They have a great rapport, and it's truly wonderful to observe them learning together.
I imagine you are envisioning those sorts of cooperative opportunities for your children in your list of advantages. You're imagining them sitting together, working together on assignments, bouncing ideas off of each other, helping each other. You didn't mention their ages, but perhaps they already work this way together at home, which is why the idea appeals to you.
I'm not trying to rag on traditional classrooms, which have their own set of advantages, but traditional classrooms provide shockingly few opportunities for cooperative work. Everyone is expected to earn their own grade separately. We did a little over a year of public school for my son, who is extremely social, and it was very difficult for him to sit right next to other kids and yet be expected to work independently.
If his sister had been in the classroom, it would have been even more difficult, and I imagine the result would be the teacher separating them. In a situation where the kids are academically mismatched, that separation would be even more likely. That lack of opportunity for social, cooperative work is a major reason we pulled him out of public school.
In summary, the advantages of having your kids learn together far outweigh the disadvantages, but classrooms are not typically structured to enable those advantages very well. Before you do it, you would have to make sure your children's teacher and the administration are fully on board with your reasons for doing so.