So, I understand in TES lore the mother passes on her race. If the mother was high elf the child would be high elf, if the mother was a Nord the child would be Nord, and so on. My brother and I have created characters that are brothers. Biologically. We mimicked each other's character's looks. Anyway the issue is this. My brother made a Nord and I made a Breton. I guess we could always pretend his Nord is a Breton, but we kind of don't. So we were wondering, how in TES lore is it possible for brothers to be bloodline brothers when they are two different races?

This link seem to tell relations of the various folk of Tamriel =)
http://elderscrolls.wikia.com/wiki/Tamriel

Or a direct link for Breton
http://elderscrolls.wikia.com/wiki/Breton_%28Elder_Scrolls%29

Same father, different mothers?

Yeah, that could work, half-brothers is a possibility.

Bretons and Nords live in provinces that bordering. Bretons from High Rock and Nords from Skyrim.

That's what we were thinking. But so cliche and overdone.

Sadly, they can't be geneticly brothers if they don't have same mother or father as Breton seem to be a totally different kind of people than the Nords.