Sense Mako and Bolin are brother gets one is a fire bender the other is a earthbender what do you think of that?

I dont completely understand. Do you mean that since they're brothers how is one and earthbender and the other is a firebender?

Well, one of their parents could've been from the Earth Kingdom and the other from the Fire Nation. I always thought that something happened between the Fire Nation parent and some bad guys that eventually they killed their mother/father. Sorry, I don't remember with one was killed. Or was it both?

Or, they could've just had different teachers. One taught Bolin Earthbending and another taught Mako Firebending!

Thats all I got...

Since Republic City was made to join all benders, they could have both traits of earth and fire bending in their bloodline. Such as their mother a fire bender, and their father an earth bender, vise versa or even for the grandparents. For example, someone's parent's both have brown hair but they end up a red head because of their grandmother.

i'm pretty sure that one of the parents was a fire bender and the other was an earth bender.

Because their mother and father were earth and Fire benders.

A firebender married an earthbender. Duh.

Well, it might be the fact of genetics, one parent fire, one earth, even that they we're given different abilities for their future.
Or plot convenience.