Bending ability isn't as straight cut as it seems. You don't need two parents with bending abilities, or even one parent honestly. How it works is the bending ability starts at 'x' member of the family. From that member on, any children of their children of their children have a chance to be a bender. So for example, say 'x' member is a bender. They have four kids. Only one of them is a bender. Ignore the bender. 'y' member is one of the kids without bending abilities, they get married to a non bender, who's never had a bender in the family line before, and have a kid. That kid has a chance to be a bender of whatever element 'x' was a bender of.
So, using Mako and Bolin as an example, it's entirely possible for multiple bender children of the same family to be benders of different elements. All it takes is having two different bender ancestry lines of those elements somewhere down the line.

BUT!
This also plays into sub-elements!
You might be thinking "but can't any bender learn to bend sub elements"
No. Bolin couldn't master metal bending.
But, he could easily bend lava, learning to do so by accident. The criminal bender, that had spent years learning to do so, was shocked Bolin could do so in an instant.
Now, you might be saying "oh, it's just a fluke accident."
That's not the case though.
Remember his brother is a fire bender? He has fire bending in his blood, so lava bending, which as we know is molten rock. Fire, and earth as one. Naturally, having both those elements in his ancestry, it was easy for him to do what takes normal earth benders years to master.