If I had a brother who also played the game I'd avoid them in (non team) games - simply to avoid the possibility of any suggestion of impropriety. I think I've mentioned it before, it would be a case of being "beyond reproach". I have no specific reason to suspect that you configured your races to work together, but if I had been doing that they wouldn't be a perfect match - that's too much of a give away to the host...
It's not simple, it's not clear cut - and it NEVER can be (unless you collaborate to make races which deliberately don't work well together, and why would you do that?)

We couldn't avoid a game with brothers, because until 2401 we didn't know

In this particular game I don't remember reading someone state in the starting thread people from the same family (from what I know, you are brothers) would play the game. I found out on year one on the game screen. This fact hit me very hard since never a single time something good ever got out of this situation every single time I experienced it in game and it was even detrimental to the game... I did and still do seriously consider creating and using a personal policy not to enter such a game again.