Mangetout, I understand that you feel strongly that their behaviors are linked and their points should be as well, but let me add another voice to try to persuade you otherwise.
My younger sister was 4 years younger than me. She thought it was great fun (perhaps a sign that she had power) if she could get her older brother in trouble. She wasn’t really impressed with being in trouble herself. So (from a rational standpoint), getting older brother in trouble = Great Good, getting in trouble herself = minor bad, and if they were connected, the sum is still positive - getting us both in trouble was fine by her. Of course, that’s if she reacted rationally - if not, she just emotionally enjoyed getting us in trouble, and didn’t worry about future consequences.
For my part, anytime that we were going to get joint rewards/punishment, it was not worth my time to make an effort. I quickly came to realize that my sister would just counteract anything I did, so it would be a poor investment of time on my part to put extra work in.
On the other hand, there were times when we were each judged on our own behavior - generally, when my parents could witness us separately. In those cases, it was worth my time to behave and go the extra mile, because it would get noticed. By the same token, my sister at least misbehaved less, since it wouldn’t get me in trouble, so there was no fun in that. Overall, a net positive result in both our behaviors.
I loved my sister dearly, but honestly, she didn’t care what my parents thought, and was more than willing to take me down with her.
In addition, there is a more general message you are sending - if they are responsible for each other’s behavior, you are delegating that part of your parenting to them, which (I think) usually means giving the older one more responsibility. One child is not necessarily ready to take care of another (especially with a 3 year age difference). I know that a few years ago, my mother turned to me and said, “Don’t you think we did a good job of raising your sister?” Well, no, Mom. :rolleyes: For better or worse, I was only a kid, and don’t think I get the credit or the blame for how she turned out.
Take this as you will - they are your kids, and you know them better than anyone - but in general, I don’t think that group consequences for behavior are effective.